Not all transwomen have penises, it's the assumption that just because they're trans you know what their gentials look like that bothers people.
Not necessarily. The people I've seen raging about it are raging about it because they're translesbians and are angry that cislesbians aren't interested in them due to the fact that they still have their penises. Those ones refer to cislesbians as genital fetishists and claim that their orientation isn't valid because they should be attracted to all women and are being transmisogynist by not wanting to look past the sexual incompatibility.
There are genuine arguments to be made but most of the ones shouting about it the loudest are only doing so because they think they're entitled to sex and relationships with people who don't want them.