How can my girlfriend make the right choice?

Just be there waiting, it takes a while to contemplate that you know? So give her the time she needs eventually she will come up with an answer. Maybe though you should tell her or write her a letter so you are still giving her the space that she needs how you feel about her and how you made a mistake and I'm assuming you aren't interested in guys anymore from that experience so just explain that to her. Such that you will be there for her and her kids and so on. Just know that you haven't been with her that long so she may be a little apprehensive because the relationship is still new and already something drastic happened. But again, just be there, tell her how you feel otherwise there isn't much else to do, it's her decision at this point so give her the space and time she needs.

You have to persuade her that you will be a reliable partner. What if instead of a gay experience you had had a heterosexual one. Would that have made things different? Faithfulness is not correlated with sexual orientation. Let her know how you feel.