It has become trendy to have sex with straight men. Some guys think that married men only have sex with their wives, so they probably have very low risk for HIV/STDs. Other guys think that straight men don't get HIV.
Men, whether married or not, get HIV from HIV-infected men and from HIV-infected women. They also get STDs. If a man is married and having sex outside of his primary relationship, it is likely that you are not the only person he is having sex with outside of his primary relationship. And if he hasn't disclosed his sexual encounters outside of his marriage with his wife (who he's often in love with, financially interdependent with, etc.), it's unlikely that he's going to disclose his sexual encounters with others to you.
Some gay and bisexual men think it's hot to "seduce a straight guy" or that it's safer to hook up with men married to women. It's all fantasy, and guys sometimes use these fantasies to trick potential sex partners, especially on the Internet. Is that guy online really "looking for his first encounter with a guy?" Has he been "looking for his first encounter with a guy" for a while now? It's important to consider that people may not be representing themselves truthfully online, and that people take advantage of popular fantasies to make themselves more desirable.
Straight or gay, married or single, bi-curious or discreet... be sure to play safer.
Also, a good rule of thumb: if he's barebacking with me, he's probably barebacking with someone else.