Maybe I'm too tense, maybe the cocks and dildos are too long, or maybe I'm just weird. It's happened both with women with strap-ons and with men with live cocks. You're not weird. What you're describing is pretty common, actually. Often, physical discomfort is due to not feeling completely relaxed at the time of intercourse.
Most people find that they prefer certain positions when being anally penetrated.
Generally, the more comfortable you are at the time of intercourse, the less queasiness you will feel the next day. It's important to take things slowly, as it were. In fact, many folks need to practice self-stimulation before attempting partnered anal sex. There is no reason that anal sex should hurt the receptive partner. This is nonsense.