Again, sexuality is pretty simple. What you're attracted to is all that matters especially since the human brain is such a powerful tool.
If the dude isn't attracted to dudes, he's straight. There are gay dudes who have kids, are married, and have never fukked a dude but they're still gay if they find themselves sexually attracted to men.
How else can you have sex with someone if you aren't sexually attracted?
Last time I checked, your dikk don't get hard unless you like what you see.