feel uncomfortable with the idea of joining any group which excludes certain people based on their sex, because I consider women and men should be treated equally.
I think this is a ridiculously simplistic way of thinking and the fact it was compounded further in the thread to this Has feminism changed to mean hating men and not wanting to spend any time in their company? makes me question your motives.
I'm wondering what level of thinking leads you from
IF women sometimes want to spend time in the company of women alone
THEREFORE they must hate men.
It isn't remotely logical.
I spend time with just women because it is a great atmosphere, entirely different from mixed company and hugely different to only men. We can speak more freely about issues affecting women and indeed many other subjects. I'm not sure what is wrong with that - I'm sure some men enjoy time in the company of men exclusively and that isn't a problem to me either.
How bizarre to assume we must be in mixed sex company at all times or it seems exclusive and we must hate the other sex 
I personally, prefer the company of women in general it would be a reductio ad absurdum to conclude from this that I hate men.
I think it is useful, helpful and positive to have groups for single sexes - consider support groups, there may be things men are not comfortable to discuss in front of women, the presence of women may mean they won't share and benefit from support. I know that this is true for many women and can even extend to activity and hobby based groups. I really don't see the issue at all - most of all I think if you do have an issue with it you can go and find a mixed sex group instead of complaining about women or men meeting alone.
(Mind you I don't see quite as much complaining done about men only groups and I can't think why
)