What’s with companies asking sexual orientation on applications?

The non-creepy answer is that companies want to make sure they’re getting a diverse set of applicants and without asking they can’t find out. They’d want to track that in case they’re doing something unintentional or not that stops people from applying.

Whether this is the best way, should be allowed and so on is another thing though! In particular because it gives cover to people that would use it in a discriminatory manner.