Homosexuals should be able to teach, definitely.
Pregnant mothers, regardless of their marital status, probably should not, especially by the third trimester. I would assume this is common sense.
My eighth grade Spanish teacher taught while pregnant.
When she was about to have her baby, she simply left for a few weeks and then returned.
No harm, no foul. Not a big deal. If they want to, it's not a problem with me.
I made my post under the assumption that pregnant women, especially in the later months of the pregnancy, get morning sickness often and can't deal with stress, as well as having mood swings. If this is not the case for the majority of pregnant women, I'll retract my statement... but all of the (admittedly few) pregnant women I have spent time with were definitely not able to teach under their conditions.