There are not many people in my experience who can be monogamous in n a relationship from 17 yrs old. That's my observation if both women and men, though men seem even more inclined to cheat.
Thats why I think it's a mistake for people to get into steady/serious relationships in their teens or even early twenties. Of course there are some people it suits, but there are plenty it doesn't.
Your fiance is clearly someone it hadn't suited. He's been caught four times now (is that right) tapping up other women and realistically there may be other times you haven't caught him for.
A person with integrity and without double standards would be honest and end he relationship or agree an open relationship or agree for each of you to do your own thing and the reunite if and when it suits you to do so (which often doesn't happen). There are definitely a shortage of people like that around, and again your fiance isn't one of them.
So .. guy can continue the relationship and hope that his attempts to sow his wild oats are over (but at 25 they may not be), accept that he has average of below average integrity, and accept that it may not just be wild oat sowing but may be his character.
Or you can agree to end the engagement, have a break and do your own thing for a while if he can be honest about whether he truly feels he's ready to be monogamous, and see how you're set at a later point. Bearing in mind that you may get involved with other people, those relationships may become serious and you may not get back together.
Or you end the engagement and relationship, on the basis that he's repeatedly tried to cheat (maybe has), has low integrity and honesty, it's possibly his character (not just the lack of realism behind two people getting into the life relationship at 17 and not being with anyone else) a d that maybe with the precedent being set - of him looking & trying with other women and you forgiving ; he'll think he can get away with it in an ongoing basis - esp when you're "stuck" with a ring on your finger and kids.