Advice please

Lost a friend or what I thought was a friend to a fwb situation. We were cuddling & having sex but he never wanted to put a label on it (red flag missed I know) . Then our friendship became just about sex. If we weren’t having sex or sexting we weren’t talking🙃. Come to find out he has a gf!!!! Of three whole years! And to make it even worse their supposed to be getting married at the end of this year. And here’s the kicker he still wants to have sex with me!!!!!! I told him no of course and he really had the audacity to ask if I was mad.

Why in the world is he wanting sex with me but “Madly in love with his girlfriend of three years!!” (His words not mine)