I think you're turned on by many complex feelings & one of them could be letting your guards off despite being vulnerable & secretly enjoying it. (Damsel in distress).

Sex is a vulnerable act, but you're turned on by your partner so much that you're willing to let him see you naked.

You fear being vulnerable from outside world & yet you're turned on by your partner taking control of you & your vulnerable side. You're in distress & vulnerable but with him, you're enjoying letting him get intimate with you. You know you can't trust any person completely, but you still trusting him with such an act.

It's just one of many other possible factors that could be interacting here.