The answer is probably in the words "I didn't object", as it sounds like you were not fully into what was happening.
In any case, this sort of thing happens to almost everyone, male or female. It's more visible with men, but women have times when they're not as turned on as others.
In future, consider doing stuff to get you more in the mood or just acknowledge that you're not there at the moment - all sorts of things can be a distraction - and do something that doesn't involve you needing to have a full erection.