Sounds like either a medical or a psychological issue, frankly. I'd get it checked out before you aren't able to get one even with a woman.

You can always rent one.

This would make a great pickup line

then go out and meet some girls!

who said you needed a gf to get laid? fuck all the biddies you want (safely, of course).

You're problem isn't getting an erection, you're problem is obsessing over another man. Maybe that's how you get erections my friend.

Being able to become aroused is to do with proper functioning of your parasympathetic nervous system. Being able to orgasm involves your sympathetic nervous system. Some drugs like, antidepressants and anti-anxiety drugs, suppress your sympathetic nervous system which can make it difficult to orgasm. On the flip side, amphetamines affect your parasympathetic nervous system and may make it difficult to become aroused. But that's only part of the story. Depression, anxiety, ADHD, and similar conditioms affect your autonomic nervous system, throwing the sympathetic and parasympathetic nervous systems out of whack. Often the above drugs help restore balance to those systems if you struggle with a psychological condition. So, take your meds if you have them, or see a psychiatrist and/or a psychologist if you don't take them or don't have a diagnosis. Not saying there's definitely something psychologically wrong with you in this regard. But Occam's razor and all that.