Um...I'm in TX too, and it's not just the heat, it's the humidity. When you are that far along, your body CANNOT regulate itself like normal. Pregnant women get heat stroke easily. With a past pregnancy, my doctor asked me if I was going outside much (it was a Texas summer). I told her no. She said good. There was a lady who DIED at a ballgame that summer! She was far along in her pregnancy and the heat got to her and she died. I'm sure that's somewhat rare, but it happens. My doctor felt very strongly that pregnant women should not be out in the heat. Ask your doctor about it. I bet he/she will have some advice. It's simply not safe.
Another story, my neighbor mowed her lawn at 34 weeks (crazy woman!). up until that point, everything had been fine. When she went to her baby appt the next day, her doctor said her fluid was dangerously low, and she had to be induced right then. He said it was because she got too dehydrated (which happens EASILY) from mowing the lawn.
You're NOT in a normal state of body. Your body works different than when you're not pregnant. It doesn't take a rocket scientist to figure that out, so hopefully your husband will get it! My sister's husband is so thoughtless of her when she's pregnant. He thinks morning sickness is fake and that women just make up all the pregnancy discomforts. It DRIVES ME CRAZY. I know some women have it easy, but that doesn't mean it's the same for everyone.
Standing for long periods is difficult. You have to take into account the big, unbalanced belly, in combo with the muscles being stretched apart, then the strain on your back. It's not good to stand up for long periods like that. I have a hard time standing in lines at the store when big/prego. I start feeling like I'm going to pass out (maybe I have blood pressure issues, I don't know!).
I would nicely tell your hubby no. If he doesn't understand why, he can read all our responses. He can know that you aren't just being a baby. It's a real concern on many levels. In the summer last year (had baby in June), I would sit out under our covered deck, which had fans, and I sprayed myself down with water. I also had a bottle of water with me. I was out there to watch the kids (youngest was quite young), and the heat/humidity was overwhelming. I could NOT keep myself cooled off. I got dizzy and felt like I was going to throw up. I barely had energy and had to go in because it was too much. This was after 20-30 minutes (Texas heat/humidity is bad!). When I'm not prego, I can handle it much better. It gets really hot, but I can manage pretty fine.
Okay, I'm rambling far too long about it. If he wants a guy's perspective, maybe my hubby can talk to him! There are physical differences for you that HAVE to be taken into account. You are not being unreasonable. I would feel really hurt and like he didn't care if he was reacting that way to you. Maybe a guy's perspective could help? My hubby gets it. When I had my first baby, he didn't quite get it (mostly it was the morning sickness and fatigue he didn't understand), but he has learned that I'm not just being a baby...and I'm so glad he has because it was very upsetting to me! I'll stop rambling. But I definitely wouldn't go...and I'd involve your doctor. Tell your doctor how you feel and let him/her inform on a medical level how a pregannt woman's body is different.
And, I'm sure your hubby is a great guy. I don't mean any of this as an attack on him. Some people just don't get it...and I hope he does!