Don't you understand that maybe people don't want to prove you wrong because they agree with you to some extent?
The mother gains no physical advantage and many disadvatages from the foetus, while the foetus gains much from the mother for it's own use and protection. In that sense it is akin to a parasite.
However, in normal usage a parasite does tend to be of a different species. When humans and domesticated animals are parasitised we tend to treat them to get rid of the parasite. We don't tend to do that with pregnant women. That tells you that no-one thnks of a foetus as a truly parasitic being.
As for parasitic twins, yes they are the same species, but the parasitic twin cannot live without the sibling, yet is slowly killing the sibling and itself.