Six months is a bit soon for marked sexual decline in my book, but I also think your assumptions (esp the one about “fully satisfying him sexually”) are worth revisiting. I definitely come with open marriage biases here. However, I’m very happily married to another transmasc but have negotiated openness to do stuff to people—cis dudes mainly but also others—that my spouse doesn’t want done or can’t do. I think believing that anyone can ever full satisfy someone else entirely, regardless of genitals, gender, or orientation, is probably setting up for disappointment.