So the threat of pregnancy is much more effective in getting people to use protection than STDs. What else is new. 
Surely you jest, bgwah. Straight people aren't (consistently) using condoms, and they're certainly not using them more than gays are, they're just lucking out. For various reasons, they do have lower risk (less promiscuous, mostly not doing anal sex, and by the very nature of female physiology,
the same person cannot be both the creamer and the creampied). But the reason for less transmission in the larger non-gay-man populace is most certainly not condom-usage.
