Is this unfair of me?

My SO has one child from a previous relationship (3yo), I have one from a previous relationship as well (1.5yo). We are expecting our first child together in April.

We usually have his son two evenings a week and an overnight on Fridays. Today SO informs me that his son was sent home from daycare due to vomiting.

Not only is my immune system down from pregnancy, but my OB suspects possible aspiration pneumonia and I'll probably be having chest x rays done soon. My daughter just had vaccines earlier this week so her immune system is down, too.

I had asked SO if his son could stay with his mom tonight to avoid risk to me, baby and my daughter. He told me no, she has plans tonight and it's not fair of me to ask her to drop her plans because their child is sick. So I said that's fine, I'd take my daughter to my mom's and we'd just spend the night there. He seemed to get really offended by this. I explained that I'm already high risk as baby has a known heart condition, I've got a history of pre-eclampsia and the flu/other viruses going around this year are really bad and I want to protect this baby from anything that could send me into preterm labor. I didn't think it was fair to assume I'd be okay with putting all 3 of us at risk because his mom wants to go out. Am I out of line for offering to spend the night elsewhere? I wasn't trying to kick them out of our house... Am I out of line for expecting his mom to take care of him when she's already been exposed? I don't know. This seems like an illogical thing to me, but apparently I'm overreacting.