Re: Sempai-Kohai relationship
"I will have to look into that. And my husband isn't just my sempai with swords, sensei considers him my sempai...period. I guess I just won't get it and I think asking how it works would be rude."
First and foremost, it is not rude to ask your sensei to explain himself if you don't understand something. It is never rude to ask your sensei a question. You should at all times feel comfortable to ask your Sensei any question. But, it is a matter of how you approach it. Don't approach him on the mat about it. Rather, ask him off the mat without other students around or paying much attention. Ask him in a way of gaining understanding.
I would harbor to guess the reasons he considers your husband your sempai despite you both starting aikido in that dojo at the same time:
1) As you stated, your husband has previously practiced aikido.
2) Your husband might be older than you even by a few months.
3) Your sensei sees your husband as more skilled or picking up aikido faster (Some sensei/schools do attribute sempai/kohai relationship to skill rather than the time you entered the dojo.)
4) Your sensei thinks the husband/male is always higher "in rank" than the wife/woman.