I'm guessing you just skipped through and didn't read the dialogue very well.
The MC was 21 and the girlfriend was 15 when they got engaged. That makes the MC an adult and the girlfriend a minor. That's illegal and makes the MC a pedophile.
The game takes place in New York. The age of consent in NY is 17. The girlfriend is two years under that. And to top that off, you don't propose to someone after just the first date. So, they were together a year or two before the MC proposed at least. That means the girlfriend was either 13 or 14, while the MC was 19 or 20, when they first got together. That is still illegal.
But apparently this is being changed in the next update (dev messed up on the calculations), so this is all going to be a moot point when that happens.
Age of consent can be an never ending discussion since it different for every part in the world, I met once a muslim girl 14 years old was already married to a 30 year old guy, though in Belgium here you can't marry under 18 without court allowance, but they allow 14 year olds having sex with max 5 year older guys. I know even girls 17 year old and still feel oblivious about sex and don't think about dating untill they nearly 30, technicaly you can get engaged at any age since it not yet the real commitment, it when you going to marry that depends on the country laws. But if look at reality, it often weird that a 15 year old in western world would already make such commitment, sex on the other hand well we have teenage mothers of 15 and younger so not all girls are daddy's little angels.
And yes I was 24 when I dated a 15 year old too, but i respected her and we had only sex when she was 19, though sadly the relation ended as she chosen an even older guy then me afterwards. Even there are laws, you can't change people wantings, but I had during that relation met 2 other girls one was 11 and trying to seduce me, which I told her she is to young to start such things and should keep on playing with dolls, sadly she didn't listen and slept with another guy, also met a 15 year old begging me to meet her and be her first, but i'm a loyal guy so told her no that I already have a girlfriend. It is what it is, but the law can't stop teenagers from wanting things and they still will do it no matter what, it on the parents to guide them and explain the dangers.
Luxury1970 you didn't make a mistake with this story, it is a possible story and it could have happened. I have met girls of all sort of ages and some where more mature then other who were older, met a 7 year old who already wanted to work in a striptease bar, met 24 year olds that were prudes not knowing what sex was, met a 14 year old that slept with a lot older guys who gave her free drugs in return, every where in the world will always be some girl that wants a ripe banana instead of a young fresh one, although most girls who respect themself aren't so easy and only give them to the man they love. So don't feel bad about your story, it is a very decent one, only prudes will complain, because their brain can't handle reality.