When men and women reach orgasm, their brains are flooded with oxytocin, otherwise known as the "cuddle" hormone. Oxytocin creates feel-good emotions and feelings of bonding. Research suggests that male levels of testosterone counteract this release of oxytocin, minimizing its effect. In other words, while women are experiencing an oxytocin "high," men might not feel the same intimacy.
Thus, while a teenage girl might feel very bonded with her partner after sex, a teenage boy might feel unattached. Of course, this isn't true in every situation. However, it's important to talk to your teenagers about how these hormones might affect their sexual experience.
Try saying something like: "I know I have taught you that boys and girls are equal, and they are. But that doesn't mean that we are the same. Men and women have different hormones that affect the way they think and feel, and some of your hormones might make you feel in love with your partner after you have sex."