BikeMexican... let me draw you a picture.
you brother
/ /
O O <-- 2 heads
x--|--x
/\ <-- 1 ass
| |
* *
You are male heterosexual. Your brother is male homosexual. You are joined and inseperable since you are siamese twins -- you can't just cut off 1 head. Therefore, when your brother has sex anally, you must also share in the experience.
I didn't find the concept too hard to grasp, myself...