Can't Take My Boyfriend's Sexual Past

I really love my boyfriend for who he is. He's nice, funny, intellectual, and an overall wonderful person. I knew he had already had sex before, I have decided to save myself for marriage (which he accepts and respects) and so that in the beginning kinda upset me, but I love him so much I let it go.

Now I hear that he's had a threesome and to me, that's just..well, disgusting. I think it's one thing to have had premarital sex with someone you cared about, and a total other to just be romping around with multiple sex partners for the sake of a physical thrill. It almost irritates me because he sees nothing wrong with it and finds it comical that I think its gross. How can I let the past be the past and focus on our relationship now?

Sexual relations are not all the same. Even though many people don't realize that. The threesome and porno-style sex is, how shall I put it, lower then when 2 people are in love. It even looks different. People in love experience something like electricity through their body, they touch each other differently, as if they exchange some unseen energies (think Aerosmith).
And when people do threesome or go to prostitute, they experience sort of relaxed release of tension, er, without thinking and feeling too much. Now, spiritually, this is indeed lower states. And it is true according to all religions, including new age and atheism which is a form of religion. Only, currently society comes to be tolerant to lower spiritual states as long as they don't do harm spiritually and physically to whoever they do sex with.
So, he is not bad, he did release his tensions, and he is good according to modern secular morals.
However, you are a virgin, and you are preparing yourself for the other kind of sexual encounter, the heightened one. Not all virgins are like you, some girls you know are pretty dirty. But you don't seem the type. Now, it is important for you to be sure that you will not be disappointed when you marry the man. People who experienced the lower kind of sex can switch to higher emotions. So it's not his past. It's what he feels to you now.
To know how he feels, you can check how he is acting with you physically now. You may not have full physical contact, and I respect that decision. But you probably have those moments when you touch hands, or caress each other, or hug. Do you feel that he truly feels connection with you and you with him during those moments? Then you guys will be ok.
The whole world is in mixed states now. We are tolerant and cosmopolitan society now. We decided to go this route, and not press on people to be pious (as oppose to some other religious societies). So you should not judge him too harshly. But check if he switched to higher state of emotions with you. This was very philosophical, sorry, I hope you get something from it.

Be grateful that he told you the truth and didn't lie. (Women rarely spare men that indignity). Since he's not broken your trust, you have to ask yourself how you feel about his past and about continuing the relationship. Besides actual experiences, he appears to have different values than you, which may be a warning flag. His glib dismissal of your feelings shows a troubling lack of empathy, in my opinion. Good luck.