A couple of things to consider. Your date how has performed oral sex on men, has it been the same man or the same few men? If so, that could signal a emotional attachment to a man that would make him lean more towards being gay.
There's a big group of men, who are otherwise respectable that hook up with other men just for oral sex. Usually, a straight man that does this (read Joe Kort's web site or book) usually wants to be sucked off by the other man, he will suck if he has to and he wants it anonymous and fast. No feeling no connections. A man that wants to suck and probably swallow other men, in my opinion, has something else going on.
You would want to know if he has has healthy (missionary style) sex with several women a number of different times in the past. If you have had sex with him has he suggested anything a little kinky, like performing anal sex on you or you using a dildoe on him.
I think I have been a wonderful husband and I was so lucky my best friend was just like me but that is real rare. I would be very cautious, especially with STD's if he has been sucking off me. There is always a polygraph, and they work. Find out the polygrapher the police use in your city and if he agrees, you all decide in advance 3 or 4 questions that will be asked, like "Have you ever had anal sex with another man?" "Has a man ever performed oral sex on you?" Only consider if you are thinking of marriage and you want to make sure because if he is telling lies, it will catch him as I have a background in criminal justice. The test should cost $400-$500.