The only sort of sex where you get to decide what happens every time is masturbation.
Here, your partner is not comfortable with something - that you think it's irrational doesn't matter, you're not the one who would get pregnant - and yet you want to do it anyway?
There are plenty of alternative places to ejaculate than in her vagina. You can also withdraw before you're just about to come too, which gives more choices about what to do next.
How is she about sex during her 'between months' bleed? Is there another (additional?) form of contraception you could use that would make both of you happy?