This is a very interesting thread mm.
I have one DD and she is in Y3. Since nursery (where, coincidentally, she had about 75% boys in her group) she has loved playing with boys as much as with girls. She's pretty gregarious at the best of times.
Many boys were friends with her back in reception and Y1, but by the start of Y2 we had noticed a definite change in the class (apparently so in the parallel class too). More an more boys appeared to move away from the girls as a whole. My DD found it hard, as she had one very close boy friend and he was actively dissuaded from hanging out with her, by another boy (one of the real alpha males in the class).
Same thing happened in Y3 last year.
Gradually, she has been left with girl only friends, plus one boy who manages to stand up to the other boys and their remonstrations that he is 'playing with a girl'. They make it hard for them and are still trying to get them apart.
I've spoken to parents with older children, about this, as it's clear that in our case, it is the boys choosing not to hang out with the girls any more, rather than the reverse. They have told me that by Y6, the two genders appear to soften and get back together again, usually. Certainly by Y7, they are saying that DC are once again more likely to have a friend of the opposite gender.
That said, many boys in our current class have parents who seem to actively encourage boy only friends/boy only parties and play dates etc. There are parents too, with only female DC, who are the same with girls, excluding boys from the outset.
We, like others, have always encouraged our DD to play with whom she likes, irrespective of gender. Character is more important when there are little anyway. Buy currently she is also more and more disillusioned by the 'mucker-abouter' kids in her class, who atre predominantly and almost always the boys.
Who knows how things will progress for her cohort - I was educated in a similar primary to hers (co-ed and state) and they were some of the happiest days of my life. I then went to an independent girls' school in London which was sheer misery, in comparison. I could only stomach 5 years and went back to a co-ed sixth form as soon as I could.
The thing I remember best is the intense bitchiness. I think I was also unlucky, in that I had a particularly bitchy class of girls to contend with, but this can happen anywhere and in the state or private sector.
I do recall, with some dismay, realising that girls tended to behave better and ***** less, when they had boys around.
This is precisely why my DD will go to a co-ed secondary!