[potluckyears]

The stereotypes associated with being a woman in tech trend towards the negative. That means that having other people signal that they see you primarily as a woman first instead of an engineer can be uncomfortable in a professional environment, even when the other person isn't bringing up those negative stereotypes themselves and may even in fact be complimenting your qualities as a woman. Those negative stereotypes exist and people are aware of them, so being signaled that your colleagues are seeing you as a woman first instead of an engineer feels like people are perceiving you in certain ways before they even get to know you and your work.

Basically, if on Day One of a new job your boss says, "I am viewing you as a woman rather than an engineer," given the negative stereotypes about women in tech, it adds a lot more discomfort, anxiety, and frustration about how others are perceiving you in the workplace than an awkward proposition would in a non-professional context.

That said, I think your question implies this is a male/female thing when really it's probably more of a you/me thing. Plenty of guys would also not be okay with being propositioned for sex on the first day by a female manager that they directly report to and who is responsible for their performance reviews.