Are butt plugs only for gay guys?

Not at all.

Homosexuality has nothing to do with physical pleasure.

For men the most unused but most powerful sexual organ in the male body is the prostate gland. That's why having a finger in your rectum is so pleasurable because it puts presure on the prostate. You can also have your spouse massage the area between your anus and your tesitcals to get some of the same effect as there are many nerve groups in that area attached to the prostate.