Is it uncommon for a guy to sit behind a girl in a horse
It's not uncommon. Riding two to a horse is what is uncommon, to be honest and not really encouraged.
not really, if you don't know how to ride one.... :)
no, but you should try to sit behind him next time
no. it doesn't really matter what order you sit in a horse.
It really depends. Romantic relationship - odd
Not at all. There is no rule on double riding as far as different sexes go.
I think it is uncommon, but it sounds ok!
No I see it all the time. From professional riders to amateur.