Can A Straight Guy Carry A 50cm Travel Birkin??..

Certainly. There's nothing feminine about that size and the proportions suit a masculine frame. The only thing that will ever make it seem girlish is if you go for a pink toned leather. It's also quite heavy so a bit of muscle tone is de rigeur and screams 'bloke'!
 
Yes - and I have seen plenty of straight guys with the 50 cm travel kelly too. Both as briefcases.

My husband says he would even carry the 40 birkin.

PS Just to clarify, I am a woman, my husband is a straight guy.

No offense intended, but a lot of straight guys ask, "Would a gay guy carry it?" If the answer is no, then they will pass on it. I am not suggesting that all gay men have good taste (that would be a gross generalization) but many straight men will regard their gay friends as the fashion experts. My husband included!
 
These don't look all that feminine - all Hermes (photo from Men's Vogue):
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