I hope you are feeling as though you have got more information today Daisy.
My post is in response to squeaky's and also partially in response to notalways.
I've also seen vulnerable posters' threads distorted by respondents' agendas, but I don't think it's anything like as one-sided as you think. I certainly think there are one or two posters on Mumsnet who have damaging beliefs about men as a sex and use the Relationships threads to further an anti-men (as opposed to a feminist) agenda but equally, I think there is a bigger group of posters who hijack threads for their own anti-feminist agenda, completely riding roughshod over the OP's beliefs and distress.
For example, if a poster is feeling distressed and hurt by the secrets her partner is keeping, usually in relation to porn or in this case, sex sites and secret texts, these posters' responses tend not only to ridicule how the OP feels ("Meh, what's a bit of porn?) but they also minimise or distort the facts, hence interacting with real women on a dating site is minimised, to "internet porn". At some point, these posters also see fit to criticise the OP for her feelings, or to blame the OP for her partner's behaviour.
So far in this thread, Daisy has been asked if she had forgotten that she had sent the texts herself (!), told that her partner is only keeping secrets because of her disproportionately bad reactions to his understandable sexual behaviour, told to "chill" and advised that her reactions might be skewed by her pregnancy hormones. In summary, some unbelievably patronising statements to an intelligent woman.
As a general comment, I think all of us have a responsibility to take our agendas out of the real-life relationships threads and onto the debating threads, but it feels wholly disingenuous to claim that there is only one agenda operating, when threads like this descend into bitter arguments and point-scoring.
I've also noticed that the posters who actually care about the OP's distress are the ones who tend to stick around and don't join the thread to have an argument, further their own agenda - and disappear when that need has been sated.