Can you tell me more about what's going on?
How old are the two of you?
How long have you been involved?
It is weird, and not proper for someone in a relationship to go on sites like match, which is people looking for mates, and adultfriendfinder which is usually for people looking for sexual encounters. Neither is appropriate and you should ask him what exactly it is that he is curious about. Ask him how he would feel if he knew you posted a profile on one of those sites? Ask him if he would give you the passwords, if it's innocent curiosity, he shouldn't have a problem with it.
The botXXXXX XXXXXne is, you need to talk with him and find out where his head is at. Is he happy with you, is he looking somewhere else, is there something he needs you to do, or something that needs to change in the relationship? All of those questions and more need to be addressed.
I welcome your thoughts, let me know if you want to talk more.