Well this didn't take long to descend into a sah/wah issue.
FWIW, I've never seen or heard people that I know being derogatory about being a sahp.
Do I feel sahp are discriminated against? I guess a lot depends on whether you feel the tax and benefit system discriminates against those who don't earn a wage but perform a role, so could include all voluntary stuff like caring, clubs, charities etc as well as sahp.
As regards returning to work, I think a sahp needs to be realistic. Several years out means you are unlikely to be able to return at an equivalent level, means your qualifications and experience may be out of date etc. There is also a perception that many sahp looking for work want part time work that is flexible re school times and holidays. All these can, rightly or wrongly, make an employer think twice.
Is that discrimination? I honestly think a lot is due to practical realities of the workplace and sometimes unrealistic expectation. Sorry.