If you don't tell Lady A that you are breaking things off with her because you met someone else, then you are lying to her and only for the purpose of keeping her as a backup in case things don't work out with Lady B--that's really horrible behavior. There's no way to keep her from being hurt if she has developed feelings for you, but things happen and the best favor you could do her is to be honest. I think you are unrealistic that you can be friends at this point--if it was really just sexual, she might agree to still see you in that capacity in the future, but since she has developed feelings (that maybe she didn't anticipate happening) why would she want to still be friends with you? Well, sometimes it happens, but rarely. You just can't have your cake & eat it too.
It's not up to you to remain friends for a couple of reason. You'll be in an exclusive r/s with a new woman and she may not want to be friends, at best friendly but not friends.
You should have a civilized breakup.
She deserves a face-to-face breakup since you've been together a few months and things are tight (even though only sexual on your part), on her part has progressed a little deeper from her side. You could meet face to face, at her place. b/c I don't like breaking up over a date. It really sets the person up for a big let-down. You could tell her let's meet we need to talk. Then you tell her you've been seeing someone else (you don't have to tell her you met this person after you met her or the gory details of parallel dating) and you would like to be exclusive with this other woman. If she gets hysterical you can tell her of your initial arrangement.
The outcome is no matter how tactful or nice you are she may get crazy. That's just how it is wit a breakup. Very few women can just stay calm and nonchalant. There's no avoiding it. Then you leave after you break up.
Then if you want to stay friendly, how friendly do you want to be? I doubt if your new gf is going to be comfortable with you being a good friend, i.e. helping her with things at her house or talking on the phone. So I don't think a friendship is possible. Even if she still wants to stay in touch you can't b/c of your new r/s.
Speaking as a woman, I wouldn't date a man who stayed in contact with a woman as friends, after they had a sexual relationship. If you remain friends with Lady A, there's a possibility you will lose Lady B, unless she is one of those carefree, ultra open-minded women. I know you feel sorry for Lady A, but remaining friends with her is not a good idea for either of you. She will never be "the one" for you, and it seems like she's more into you than you are of her. Let her go. Let her have closure. Let her move on. There are times you don't disclose all to save a person's feelings. I wouldn't tell Lady A about Lady B. I would tell Lady A that you've enjoyed her company, but now have the goal to find a lifetime partner, but unfortunately you two aren't a match as far as that's concerned. Tell her that it won't be fair to either of your future partners to stay in contact, so you won't be communicating any longer. Life is not a fairytale and full of hurt and disappointments. Be realistic and set boundaries so that your future relationships can succeed. That means leaving your past in the past.
Don't tell Lady B about Lady A. You haven't been exclusive with her, so you're not lying-just not disclosing unneccessary info that would be upsetting to her. Just like online dating. You have to assume your date is dating others, but it wouldn't be pleasant to discuss that with them. Now that you've made up your mind, you'll no longer be with Lady A and can see where your relationship heads with Lady B. Even if Lady B doesn't work out, don't contact Lady A again. After a guy broke up with me, he texted me about 4 months later. It really pissed me off that he intruded himself into my life when I was trying to get over him. Good luck.
Here is what you should do because I was in Lady A's shoes not too long ago. Basically it had started out as fwb thing...I didn't really want that BUT I did it. Then I thought we were friends. I really enjoyed his company.Anyways..that fwb thing ended in 2011 and we were just friends. Of course I fell in love with him.I had genuine feelings for him and told him how I felt about him since late 2010 but he thought I was just infatuated with him...HIS words.Anyways we were still friend UNTIL this year when I finally found out what a horrible person he had been to me this whole time..meaning in not being honest with me that he had found someone that he wanted to be in a real relationship with. NOW instead of him actually thinking that I would be happy for him..which I would have been...he felt that he would keep me on the side incase things didn't work out with this other person. Basically how I found out is that I once again fell back and had slept with him.Now we BOTH knew that we shouldn't have done it but it happened.So anyways later that same week after it happened I had asked him if I could come over..now it was to NOT sleep wit him but to hang out.I had been going through some things at the time and I really just wanted some company. Well he told me that he would be going out of town that weekend and that he would keep me uptodate if things were to change. Well I found out that all of that was a lie.He didn't go out of town that weekend and THEN I found out on FB that he put his status as being in a relationship with this other person.I was PISSED because I realized that he had met this person really early this yr and we had selt together since then and the last time had been at least 1-2 months before he had put this person as being in a relationship with him! Also in March when I had asked if we could go hang out...he had stated that he had had plans. Come to find out ..it was with that person he is now in a relationship with...OH and we had slept together in March...so BASICALLY what I'm saying is that WHEN you know you like someone better...you need to tell that other person right away because it's not cool to keep that a secret. Of course the person will be hurt...BUT if she is your friend ..she would be happy for you. The reason I was VERY hurt was the fact that this guy KNEW he had wanted to be in a relationship with this other person YET during that...he had slept with me! Instead of treating me like a friend and also trusting me and telling me that truth...he lied to me.He really hurt me so very badly because I would have NEVER done that to him.Infact after I had told him I didn't want to do the fwb thing anymore...in 2011 I met a guy that I really liked and I told my ex-fwb because I was happy that I had found someone that wanted to be in a relationship with me and I didn't think that I would stop being friends with my ex-fwb because he didn't want to be in a relationship so I thought he would be happy for me.All he said was that he wouldn't know how to treat me anymore. I just told him..treat me like just a friend.I didn't think anything of it then...but now it all makes sense. Like someone said recently to you..he just wanted to keep me on the side incase it didn't work out with the other person. That is very wrong and hurtful to do to someone..You have staed more the once that you want to be in a relationship with Lady B.You KNOW this..You FEEL this...so that's how it is.If you even care about Lady A like you have claimed... you have to tell her. She will respect you MORE if you tell her the truth THEN if you still keep her around and NOT tell her.All the while she is thinking that she can still sleep with you while YOU know that you don't want to anymore because of your feelings for someone else. Please tell Lady A. She deserves it.Like I said before...yeah she will feel hurt BUT if she is your friend...she will just want you to be happy..even if it's not with her.Please do the righ thing.
You do not need to end it with either woman. Just do not lie. Enjoy this time. Use it as a learning experience. Clingy does not mean anything more than clingy. Do not tolerate it. It is easy to want to follow cultural teachings but they are wrong. The best communication is to point out that it seems to you that she is getting clingy. that is the only thing you need to say.
Now my experience is that when you are upfront at the beginning there is less problems. I have had FWB's and a roommate and others all at the same time period and never had problems. I am friends with most of them to this day years later. Although most of us have moved to different cities.
Neither Lady A or B should tolerate deception. How would you feel if you had feelings for a woman who was seeing another man? Playing games only leads to bitterness, dissapointment, or hurt.