I've been living with my bf for seven years.
It is called gaslighting. Look it up.
He has boundary issues. If you are in a long term relationship and he is using "friendship" as his excuse to step out . You need to a real eye opener...Read the book NOT JUST FRIENDS BY SHIRLEY GLASS. It is for married couples but it gives excellent advice for keeping up boundaries in a relationship so that no third parties are allowed into the relationship. Boundaries should be firm and defined.
He is stealing time thought and energy from your relationship and investing in another... you find evidence that they are in a relationship ...and you were for all intent and purpose
you need to tell him your feelings. Lay it all out and let him know what you think.
If he is cheating on you, he is not worth it. but at least you can get your thoughts on the table.
Hang in there and let us know how it goes.