Part of the reason guys are typically so unwilling to explore the same sex is what society has placed on being gay. How many times growing up do boys call other guys "faggot" or "you fag" or any other term not really calling somebody gay, just using the term as a bad thing. You never hear girls doing that to other girls.

Also, here is another big reason in my opnion. Guys love the thought of 2 girls having sex. VERY few guys are not atleast a little turned on at the thought. But on the opposite end, girls are typically not turned on at the concept of two guys having sex. While a few guys may not care much about 2 girls having sex, many women are repulsed by the idea. Just my thoughts