I'm a 50-year-old married dad, been straight all my life (until recently) and would have said that that would never change.
But about 2-3 years ago, I began fantasizing about cock. Not other men, mind you. Just cock. How they look. Different sizes and shapes and colors. How cool it is when you see a big, powerful orgasm.
These thoughts turned into thoughts of what it might be like to watch another guy masturbate. Which morphed into fantasies about mutual masturbation. Then to giving a handjob. Then finally, to sucking cock and swallowing.