I do not like the advice I'm about to give you, because it sounds like you love your boyfriend and otherwise have a good relationship....However, I do not think you can be happy with this man in the long run. You are not even married (yet?), and your sex life --a vital part of a relationship-- is unsatisfactory. The fact that you are posting on this forum means that you consider sexual intimacy to be important in a relationship. That is not true for everyone (including your boyfriend). Some people simply don't have a very strong sex drive, and consider sex to be a chore. The key is to get lucky enough to pair up with someone whose sex drive matches yours. In the beginning (first few years of dating/marriage), things are always hot and exciting. It's not until 5-10 years in that the marriage is supposed to get boring and sex drives dwindle. The fact that sex has never been a healthy component in your relationship is a terrible sign of things to come. You should not be thinking of cheating on him just from a sexual perspective. That's not fair to him, and it's not fair to you. Instead, you should be in a relationship that does not require you to even consider looking outside the relationship for sex.
You'll probably get all kinds of advice that maybe he has low-T, etc., but in the end do you want to spend your life frustrated and hoping you can fix a man who does not consider himself broken? Believe me, it is a frustrating position to be in. I know, because I'm in a 10-year marriage, with a woman I love and am very committed to...but she is like your boyfriend --just does not care about sex at all. She has always been this way. I wish I had known 10 years ago how frustrating it would be to live with a spouse whose sex drive did not match mine....