Am confused

i am now dating a very good friend of 5years for 2months now,we were in love with each other even before we started dating almost 2months ago. the first two weeks of the relationship was fun,we were always together everyday but after that i started noticing that he decided to distance himself a bit.
before we started dating,he was dating a lady that has a serious relationship with someone else which my boyfriend knows and when the lady noticed our closeness she asked him to choose between the two of us which my boyfriend choosed me over her and that was why he finally asked me out.
The problem i have now is that he has been seeing the girl behind now and it seems she is pregnant for him as a result of the sex they had after we started dating and am also pregnant for him too,i didnt catch him in the act but he actually told me about his escapades himself and he wants me to stand by him that he prefers me to the other lady but the problem is that he still gives her audience and i believe she will want to tie him down with the pregnancy even if she gets rid of it just to get him closer to him.
He still wants me to stand by him but am confused because i dont know what to do and all his ex girlfriends wants him back and he weakness is that he can't just say NO to other ladies advances and believe me am crazily in love with him and i know he loves me too.

Dear Anonymous,

I am hoping that all this has worked out for you. Mike