They are taking a life. Arguing at what stage something "human" is nonsense to me given that they'll meet that criteria given the proper amount of time.
The implication that you shouldn't have sex unless you are (financially, emotionally, whatever) prepared to support a child.
Not trying to offend but while sex is pleasurable it's also the means for creation. There's an inherited risk there.
Just giving you shit. That's where you have conjoined twins and one of them is dependent on the other for some organs or functions. It's a less formed member of a pair. Often it is not a human being by my definition, but I am wondering how you would classify it. It varies a lot from case to case. Sometimes it is a lump of tissues and organs, sometimes it is in fact a human being capable of speech and such. Look it up sometime, interesting stuff.
If what you meant to say was that you shouldn't have sex if you aren't willing to deal with the consequences (either carry the child and put it up for adoption, or try to raise the child despite being unprepared), then I would agree. I took you to be saying something stronger than that, and I'd disagree there.
What if one or both parties are unable to conceive? Then would it be okay? Is homosexual sex not really sex? Is oral sex not really sex? I know, semantics and shit. I'm accused of being black and white though, so I'm asking about the grey.
Depends. Some parasitic twins are fully functioning in terms of personhood. Some are just a big mole with hair and teeth.