So interesting and so depressing. There's a big overlap here with the 'facilitated men' threads, isn't there. This is part of the opposite side of that coin - in terms of employers' attitudes and assumptions.
I do take your point though FloraFox that it can be more blatant and the 'Daddy bonus' can be just that. Maybe it is, more often than we'd like to imagine.
I recall a friend, about 15 years ago, telling me she'd just been party (but not powerful in relation to) a discussion on annual salary adjustments in a law firm. One partner said to another 'oh, so and so bloke has just had a baby, he'll need a bit extra', thus adding a few £k to said bloke's salary.
What's additionally insidious about that, is that said bloke was at an early stage in his career and that boost will have had repercussions year on year for all the rest of it (plus proportionate pension contributions). No-one gets adjusted down, or goes for a job that pays less. Any 'cost of living' increase adds a % to what one already earns, and on and on, year after year.
Job applications ask for current salary so that new employers can offer more, as necessary, within the band for the job. I've fallen foul of that, when coming from a job in a poorly paid sector, into a more senior, better paid role. I was automatically started at the bottom of the band, explicitly because it was more than my previous salary so they didn't feel they needed to offer me any extra, not because of competence or experience. So, had the second choice candidate come from a better paid role (which they easily could have, if at the same level as my former job but in a different sector) they'd have been offered more money to do the same job, possibly less well.
Sorry, that's a bit of a diversion. It serves to illustrate one knock-on effect of these pay rises though.