[nf8nnfufuu]

We don't value fields less because women enter it.

First of all, there is simple supply and demand. More candidates wanting to do a specific job means lower pay.

Secondly, you should ask WHY women suddenly want to enter some field. Very likely, circumstances have changed for that job. Maybe it now allows for flex time, or new machinery makes it safer, or whatever. I don't have a good example at hand. But I would look at that - very likely it is that thing that makes it cheaper (because it is more attractive), not the women entering it.

Another possibility could be a job starts paying less, so the men leave and the women come in (since they are less dependent on high income).

Edit: I just saw example of physicians on the net, which reminds me that women also tend to work fewer ours. Apparently physicians is one of those example where women have taken over and rates seem to have dropped. However, I know that female physicians work far fewer hours than male physicians (averages), which could presumably explain the drop. (example from Germany, don't know about physicians world wide).