Can men wear birkenstock Madrid sandals?

Yes I do and see nothing wrong with it - there is nothing gender specific about a flat single bar sandal apart from colour, so as long as you sensible and stick to Black, Brown or maybe White you should not offend anyone.
I used to wear the flat wood Scholl sandals, but to be honest they are so uncomfortable compared to the Birkenstock Madrid - I bought my Madrid's at the Birkenstock shop in Covent Garden and asked the shop assistant if it were acceptable and they said yes. The only limiting factor is size, you need feet smaller than 42!. Unless you order from Germany, or buy on Ebay, where you can get up to size 46 R.
I would like to say that the Birkenstock Madrid sandal is for men and women. Maybe the pink ones should only be worn by girls. But guys can pull off the rest of them. I frequently wear the white ones with smart denims and a shirt or vest. I see nothing feminine about the design of a one bar sandal. Guys wear toe loop sandals and gladiator sandals too.