It's definitely more difficult at least for them to ask someone out. It can be very difficult to tell whether a person is straight, gay, or bi. Also, with the actions of some people who are homophobic, it could actually be a danger to the person to ask a straight person out, thinking he or she is gay.
Yesterday, I met this really hot guy working at a game store here, but I have no way of knowing if he's gay, and I'm definitely not going to ask. If he's not, he'd probably kill me, considering what most people are like where I live. *sadface*