Welcome SAO
Yes, I think that FastFood is right ~ the main thing is that you love each other.
Also, at the time, you knew that she was officially dating someone else, so when he came back he would have expected her to still be his girlfriend. In a way, she was. By going out with you, before splitting from him, she was two-timimg, as they call it.
It is understandable that she was confused and unsure and even that she had sex with him ~ not that I agree with this.
The best thing for you both would be to accept that, at the time, your relationship was in a state of uncertainty, because the other man was still on the scene.
Now, though, the relationship is settled & secure and you love each other.
However, it is very understandable that you feel confused, upset, angry, etc. She cheated on you and she lied about it. You are not going to be able to simply accept something as serious as this, without having some concerns.
Don't brush this under the carpet ~ it will fester. Sort it out now. Discuss it together; think about how you can come to terms with it. Don't just marry her because you loved her, trusted her and asked her before you knew the truth. Marry her because you love and trust her now. Maybe you will need to become 'unengaged', and then ask her again, when you feel ready.
Good luck!
