Stop taking your bf's around your grandfather. You're right that he is disrespectful to them and to you. Your grandfather is lucky that so far you've had polite respectful bf's who didn't retaliate when gf pinched them or said inappropriate things. Some guys would answer with their fist when getting their ass grabbed by another man, or having their sexual orientation questioned.
I don't really think what grandpa is doing is illegal.
You really don't think that pinching another man's butt - uninvited, and then saying "oh u liked that didnt u u like your ass touched" ------- that it's NOT sexual abuse? Keep in mind that the old man is in a slight position of authority over the young man, as well, with his greater years and in his own house. In my experience, that puts it clearly into felony territory. What if there are children witnessing this activity, and everyone laughs it off? What does that teach them? Women who endured this sort of thing for centuries were wrong to tolerate it, and should not. No one has the right to touch my body in a sexual manner, uninvited, and get away with it. No one.
I agree that Grandpa may not realize the implications of what he's doing, and truly may be doing it all in "fun." Maybe it was more accepted in Grandpa's day. But that does not give him a pass to continue. He needs to stop; at least the OP needs to ensure children are supervised when they're around him. I'm serious about that.
I would begin by not bringing every date home.
>>You really don't think that pinching another man's butt - uninvited, and then saying "oh u liked that didnt u u like your ass touched" ------- that it's NOT sexual abuse?
No, I don't think it's sexual ABUSE.