I am experiencing the same thing in my marriage right now. When my husband and I first started dating he was very sexual and very romantic, then somewhere along the way all of this stopped, now I am lucky if we have it once a month. A couple of months ago he admitted to me that he was sexually abused as a young teen-ager and that could be a reason why things are happening the way they are, but just because your boyfriend is acting that way doesn't mean he was abused. But whatever the case, don't let his actions hurt your self esteem, when this all started happening with me I was wondering what's wrong with me, but there is nothing wrong with you or me, they have the issues that they need to deal with, and don't let him make you feel bad in the process.
Good luck