and also I don't think it's about "rubbish" parents at all. I have 3 boys, they know I have a period each month (well DS3 doesn' yet- he's not asked anything about it yet). As it happens I'm single parent who isn't at all averse to the idea of potentialy in the future having another partner and maybe even more children - so we've discussed parent break-ups, step families etc (they're very keen for me to find a "nice man and have another brother or sister.......but they'd really like a sister"
). Growing up I only knew one boy who lived in a LP parent family (there may have been others but it was never talked about - was too much of a "stigma" attached to it back then). My parents stayed together "for the children" (a common belief back then that it was better to stay together for the children than to end a bad relationship). They never had any need to talk to me about single parent families, I didn't know any (apart from a vague knowledge of a boy at school who was teased mercilessly by many about it
) and they didn't know any (apart from another friend whose mother had died - so they talked to me about that situation - but not the general picture). There was no need in their eyes to do so.
We didn't (afaik) know any gay or transgender people either - so again not something I asked about - or that they thought to mention to me (I first learned about those from a magazine......),.
It will be natural for most parents to talk about sex and relationships in terms of what they know and have experienced, or what their children ask about. If the child never asks about something which isn't found within the parents family and social group it's highly unlikely that parents will think to tell them about it.
Every family will have it's own unique outlook and experiences of sex and relationships - and that is what they will teach their children. For some - it will cover the entire spectrum - others will have a much smaller experience to draw on to talk to their children about.
Sex and relationships education in schools offer children access to the entire world of sex and relationships (both intimate and non-intimate) so they can hopefully be more tolerant and understanding in the future.
In the same way that telling an enquiring 7yr old (as I did recently) that yes "2 boys can love each other" isn't going to turn him gay - neither is educating children and young adults about relationships and sex (and really if you look at the ncb website very little of it is about the actual "sex") going to make them rush out to have sex.
Our of interest - how do you feel about educating children about drugs at the upper end of Primary school and in senior school???