This is what I say to that. Being cisgender means that you are happy in your biological sex. There are plenty of cisgender lesbians and cisgender gay men. Transgender means that you are not comfortable with the sex assigned at birth.

The difference between being transgender and intersex is that in the first case, you seek to change from the sex you were assigned at birth, and in the second, you have the sexual characteristics of both sexes, and you may choose not to have gender affirming surgery.

You can be cis-het but not interested in having sex. And, it doesn't mean that you should be in a convent, if you say, "I have a partner, whom I love, but we don't have sex. Not because we have 17th Century puritanical views, just because we don't like sexual contact."