I don't think weight is a protected category, so yes this is fine. For example insurers will discriminate based on weight. The military discriminates based on weight. Hooters discriminates based on weight. Etc.

Well I hope we don't end up in such a world. Even if I personally won't be affected negatively. Sounds incredibly distopian.

You already do. Just because nobody writes an articles talking about how they try to avoid hiring overweight people doesn't mean it doesn't happen.

Or to be sterilised so they can't get pregnant or make their partner pregnant. Having kids leads to missing work after all.

An employment policy interfering with employee pregnancy, in particular, is illegal (but only because it was explicitly made so in 1978 - https://www.eeoc.gov/statutes/pregnancy-discrimination-act-1...).

In general, when an employment discrimination is not specifically forbidden via the Civil Rights Act (or interpreted as equivalent to something explicitly forbidden by the Court), it is legal to discriminate based on it in the United States. Beyond that, the Court and the legislature are (from a political philosophy standpoint) generally willing to let individual employees and employers (or employers and unions) hash out the messy details.