Teenagers do lots of things they shouldn’t, including having sex, but good parents do not condone and facilitate such behaviors. That’s simply not how morality is taught. Also, allowing this would put undue pressure on your daughter to remain in a relationship with him if and when her feelings for him change, which is likely. Have you considered what happens if and when they break up? What if it’s a nasty break-up? Are you going to boot him out, or allow him to stay when your daughter doesn’t even want to see his face?