I personally would NEVER accept the "I can't get it up with a condom on" excuse from a partner. A former partner used that as an excuse to break a fluid bond agreement and it resulted in me mistrusting him (for a good reason, since he promptly contracted an STI). A little lube inside the condom and practice masturbating with one on can work wonders, as well as knowing that you're helping protect the people you love by wearing one.
I don't know your story. But, both things could be true. Your partner could have had real trouble staying hard with a condom AND had the misfortune of that coming back to bite him (and, you potentially).
Treating it as if it's not a real problem is not helpful (sorry if that's not how you feel - just taking that from the language of "excuses"). That doesn't mean, however, that you have to simply accept it and have sex without a condom.
I have over periods of my life not been able to stay hard with a condom. It becomes a frustrating dance of - "take it off, masterbate or whatever, get hard, condom on, go soft... rinse and repeat until your patience or time runs out." My guess is that during those times, there was a real physiological cause (could by mental too, I suppose). But, those two periods in my life have given me real sympathy for this as a problem.
Men, of course, can use it as an excuse because it does not feel the same and they prefer bare exclusively. That's a probelm too. Just a different one.
I personally have sampled nearly every condom made to make this less and less of an issue. For me, condom choice has made the most difference. Most everything else didn't work (including lube in the condom).
Even now, when it's much less of a problem, I will choose most often not to have sex (at least penetration), if my preferred condom is not available.