"We're really happy together and now we're ready to have a family."
You'll be divorced and hating eachother within a year and you'll damn any kid that you two have together to live a life of pain and shame.
And I don't say that because of my general distaste for marriage but rather because if you two really loved eachother and were really head over heels in love with eachother at that she wouldn't of felt the need to go bang other guys.
From reading your post especially that little line about you wondering out loud as to whether 5 guys was all she banged I get the strong impression the breakup to go fvck other people was more her idea than yours and I'll believe that even if you come back and try to tell me that oh no the breakup was mutual so yeah you can save that reply for someone whose retarded enough to believe it.
As for the youth, the being young excuse as to why she had to go and bang other guys you can throw that right ontop of the pile with other excuses such as "well I was drunk which is why I banged so and so" or "I made a mistake" etc.
The point being that excuses are just that.
Excuses.
Almost no excuse is ever valid.
Excuses are evasions of people simply not wanting to take responsbility for their decisions and what they have done consciously.
So youth is not a valid excuse of her wanting to go and sample other c0cks it was just an excuse to do what she wanted to do more than commit to you for life.
Strange c0ck was more important to her than creating a loving union with you or loving you and thats something you should and probably will think about every day of your marriage should you be stupid enough to go through with it.
There have been people out there (though rare in this day and age) who get married at 18 and stay with their partner for life as being in a committed union with the one they love was more important to them than going and fvcking other people.
This girl's first priority wasn't you but rather having other guys slap her face and ass with their c0cks and bust loads all over her body and jiggle around the head of their c0ck on her tonsils and maybe esophagus even.
Think about that sh!t when you say "I do" and slip the expensive ring on her finger.
Think about her deepthroating someother guy and loving every inch of it while her mouth strains and her eyes tear up.
"Given that you were in my situation would you still get back with her regardless of the number of guys she slept with?"
No.