What does the face look like to you? It clearly isn't disgust or denial.

You might perhaps say she's terrified, but that's just being nervous taken up to 11. And it does nicely match up with her actions .. slowly lifting that skirt unsure of whether she'll really be accepted or ridiculed and rejected.

This also goes for girls crying in h-manga which I never understood.

It's not limited to the h-stuff.

Crying is just an outburst of emotions, and can feel quite relieving and wonderful given a proper context. Look at e.g. the scene where Light gives Misa that first hug of theirs in Death Note. The reason Misa continues to have tears in her eyes is clearly not discomfort.

Anime is all about taking things over the top (from what little I've seen so far). Why should drama and emotions be any exception to that?

That's of course not to say that there aren't other reasons why people end up or pretend to be crying in porn, or that there aren't people that enjoy pushing others or being pushed themselves beyond their limits... But tears are at best only a necessary indicator for that.

And even then, we're still a long long way off of non-consent-territory. After all, humanity is all about pushing beyond established limits - be it flying to the moon, taking an arm in your rear up to the elbow, or getting your body hair waxed off. Tears just show that it's very much humans that are doing those things, not that they are being pressured into doing them.