Well, I think you should get it back ASAP and you are absolutely in luck here. You're not inquiring about an ex-boyfriend who has your sex tape in his possession. If he was your ex than I'd say good luck and don't get your hopes up. However, that is not the case as the two of you are still together.  Phew, because the chances of getting something like a private video back from an ex is near impossible, mainly because he has no reason to appease you.  Fortunately, your boyfriend probably wants to keep you around so you'll most likely be successful in gaining control of the tape if you gently ask for it back.

Here are a few things, though tough to hear, that you need to keep in mind.  When broaching the subject, you need to make something very clear: this is not a trust issue (although it kind of is).  Explain that it's all about your comfort level as opposed to fear of a leak.  Tell your beau that you would hate for the tape to fall into the wrong hands and have someone see you in such an intimate moment.  More often than not, the man involved always seems to take possession of home videos, which has always astounded me.  It seems that as women, we have more to lose if word got out that we participated in such an explicit video.  For various reasons I won't bother going into at the moment, women are looked at differently in terms of their sexuality.  Men are given certain freedoms we women don't get, and that's precisely what you need to explain to him.  

While there seems to be a current wave of sex tape releases in Hollywood, unlike celebs, one thing you don't really have to worry about is the tape hitting the Internet for everyone to see.  Lest we forget that you are not a celebrity and therefore there just isn't any money in it for you or him.  Most stars are targets because of their public standing and people's appetite for a closer look into the lives of the rich and famous.  Your tape, if ever released, would be lost in porn oblivion, probably never to be seen again.  Or so we can hope.  It's just difficult to tell, but that's assuming someone would go to all that trouble to have someone else view it.  Most guys just hang on to those tapes because they can and might want to take a peek later on.  Very few have ulterior motives. 

In the event that he refuses to return the tape, you need immediately drop him.  You can always take some kind of a sneaky route and have someone steal it back for you but that is treading dangerous territory not to mention illegal.  I mean, do what you have to, but try and do it within the law.  Although I really don't think it will come to this, especially if you are in a loving relationship.  A boyfriend should understand and grant your wish.

My advice to women that may be entertaining the idea of making a tape is film in the comfort of your home and keep the footage.  Don't for a swift second let it out of your sight because you never know how the relationship is going to end.  People bitter over time so don't be afraid to take control of the situation.  And it's easy to do so as most guys like the thrill of making a tape and will agree to your rules as long as they get to make it in the first place.  Or alternatively, if you do not wish to have people see you in the act, don't film it at all. 
 
And one last thing: hope he didn't make a copy.