If you're going to try to differentiate a fetus from a parasite on the sole basis that it happens to be the same species as its host, then you've tacitly conceded the point. A parasite is as a parasite does, and you've basically stipulated that a fetus does exactly what a parasite does, it just happens to be of the same species as its host.
otherwise, human reproduction doesn't work, and that's all we're really here for. that's kind of the point of sex, which you're completely missing, even if sex doesn't usually result in pregnancy. that's what those fun parts are for. your argument is ignoring basic biology.