I'm having the exact same problem, and it's incredibly annoying.
I want to have protected sex, because, frankly, it's fairly stupid not to at my age, and being the product of unprotected sex at a young age myself, I'm not to keen to ruin my long-term plans like my parents did.
However, any time a condom goes near my penis, it goes soft. I can be extremely horny at the time, and not even really thinking about the condom because my girl is putting it on and not me, but it still happens.
The only times I've had sex have been unprotected (other than the girl being on the pill), simply because it seems to be the only way for me to have it, but I really don't want to make it a habit, as going to the clinic is a real pain.
I've tried putting them on myself as practice several times, but the same happens. I find them really uncomfortable to put on, as I have a wide girth size, and as soon as it starts to struggle to go on, or once the rim of the condom starts to constrict my penis, it goes floppy.
Is there anything I can do?