Why are gay men on heterosexual dating sites?

I've had this happen to me twice in 2 days!

I am female on a dating site looking for men.

I've had 2 men from the dating site act as if they're looking for a relationship etc and then when the conversations turns deeper and you exchange numbers they tell you they're gay.

Now, I don't have issues with gay people at all. Some of my nearest and dearest friends are gay but why on earth are gay guys on heterosexual sites?