Remember that some guys do prefer it, though. DO NOT be afraid to use your hands. Specific techniques in the advanced section. Even among eroginous zones, don't stick only to the penis. The cheeks of the butt, on both men and women, actually have some automatic sexual response linked to them, especially from repeated pressure.
Guys have it too, for the same reason they have nipples.
A good butt massage, or perhaps gently rhythmic pressure, can really get some guys going. The anus should be carefully mentioned here. I say carefully because, of course, a lot of guys feel uncomfortable with attention to it. But it is an erotic zone, especially because it includes the prostate gland. But more, again, in the advanced section.