Ok, well im only 16 myself, so maybe it will help to see it from a teens point of view, Im not sure ... although im a little older for my age and am still a virgin.
I know alot of people who lost their virginity at a very young age, some regret it and others are sill with their boyfriends.
It probably was right for her at the time, and although i agree that the 18 year old maybe should have thought it through more... but maybe he did, who knows.
They may have really strong feelings for one another and they felt they could have sex.
It is illegal however when people are ready they are ready and as long as she did it safely then thats the best thing.
However i don't agree with people having irresponsible underage sex.
If it was a boyfriend of hers then i don't think that it's AS much of an issue as it clearly wasnt a casual thing.
The only thing i can advise, which is hard for me as i havent had a mother for many many years, is that you're more of a friend to your daughter
speak to her about why but not with the adult 'Oh my god, what the hell are you doing, you could ruin your life' appreach cause taht will only push her away.
And talking to her about 'the birds and the beez' in the usual way probably won't help much either
Growing up is such an emotional thing for everyone and i can understand that, as a parent, you are very distressed but maybe you just need to try empathising. I think you're a fantastic mother for seeking the advice of other people instead of immediately going off on one.
Im not sure how she told you, how you found out or whatever but give her the more modernised version of 'i was young once' and maybe you'll have some sucess
Im no expert cause im definately not a mother lol... ut maybe seeing it through the younger generations eyes will help
at least, i hope it does.
She's lucky yo have a mam like you.