I see nothing wrong with your friends being in love. I don't think there's anything wrong with them curling up together, holding hands, kissing, anything like that. I will say that I could not support the two of them having sex, anymore than I would support a straight couple of that age having sex. It is highly unlikely that they will be together for life, and I tend to subscribe to the idea that the level of sexual intimacy two people should be on should be proportional to the level of commitment in their relationship. Total sexual intimacy is best saved for a relationship of total, lifelong commitment. Of course, I wouldn't condemn them for it, either, and I would hope they are provided with any information necessary to make safe, responsible choices no matter what they choose, but if they asked my advice on it, that's what I'd say