best friends dating!

ok so ive known this girl for about 7 years but when i first met her i thought she was a whore and she kind of was. slowly but surley she starts hanging out with the guys. we become best friends but i still hate how much she goes out and flirts with guys. she says she met a guy who i and everyone else saw as trouble. this guy was almost10 years older, had two kids and a girlfriend whom which he was not getting along with at the time. he gets my friend pregnant. he turns out to be a deadbeat pretty much surprisingly enough. i start helping.out.my friend and she ask me to be the godfather which i accept. slowly feeling develop and we start dating. now the kid is 2 and the dad wants wants to see the girl. we all go to a park so he can meet his daughter for the first time in about a yr and a half. now im troubled because im angry that she was so stupid and stubborn to mess around with this guy. i guess actually meeting him made me sick to my stomach knowing he has a baby with my girlfriend... i recently found out im sterile and cant have kids by the way currently undergoing treatment to try to.reverse that.. srry for the loong story but this has gotten pretty complicated. were both 23 by the way.. ADVICE PLS!!

One word for you : RUN!
You are way too young for this and you should not accept or get involved in issues that are way beyond your concern or age.
Dont do this to yourself.

its really difficult making that decision becaise were so close and we still really enjoy spending time with each other. she just got an apartment with her daughter but i just cant move in with them because im mot ready for the responsibility and problems that come with living with a two year old. dumping her would leave me without a best friend also. she is my first girlfriend also.