Note: this starts of examining the behaviour as harmless, and then explores it from the point of view that its evidence of malice.
Try not to downvote until you get to the end.
Should I take this personally?
We can't answer this. You can if you want, but I wouldn't recommend it. It might be personal, or it might just be a group of friends getting together.
Lets get a few things out of the way.
Is it sexist?
Probably not. Opportunities for men to bond now days are rare, and it may just be an attempt for some guys to get together and share a common experience. Sometimes people enjoy mixed company, sometimes they enjoy the company of the same sex. Just the existance of the club isn't evidence of sexism.
The question you really need to ask is "Do you really want to go?" and if so, why?
As you said:
There's a "boys only" group of guys who get together to meet at a bar and watch sports.
No where in your question have you expressed an enjoyment of sports, or bars. Have you previously stated an enjoyment of bars and sports at work? If not, they may have just assumed you wouldn't enjoy it. Contrary to popular believe, sports bars aren't great places for business. They are loud and distracting, and only conducive to very short conversations and most likely if they are there to watch sports, non-sport talk will probably be frowned on.
So what can you do?
If it is a sports night, and you want to go because you enjoy the idea of talking about sports with work colleagues, start talking about it at work. Bring up the games you watch, where you drink with other friends, events you are interested in. If you demonstrate that you are interested in coming for fun, and are knowledgeable on the topic, its likely since its new, that someone will just invite you out of habit of conversation. Go, enjoy have fun.
What if you don't like sports, but don't want to be excluded?
Firstly, I'd recommend against complaining to HR, at least straight away. Going to HR is the nuclear option here. If it is just a guys sports night (which again, isn't illegal or immoral), and you get it shut down because you can't go you will become the enemy - and as unpopular as this statement might be, some of the guys might be right to feel that way. Getting an event shut down, because you can't (or don't want to) go isn't fair for two reasons:
- If it is just a friendly group, you basically make it impossible for them to be friends. Its well documented that after a certain age, men experience difficulty in making and maintaining friendships. Men are also shown to act differently when in single-sex rather than mixed groups. Taking away an avenue for a group for men to be friends, taking that away may make people angry.
- If it is a networking group, it is sort unfair to guys who may not have other avenues to access management. Most formal "mens-clubs" are now not allowed, and contrary to common believe, most men don't have access to the all powerful. As a woman, you have access to "Lean-in", the Ada Initiative, the Anita Borg Initiative, and many more groups specifically targeted to helping women advance. There are very few mens-groups for those starting out, taking that away may make people angry.
As an alternative, consider doing what the boss said and starting your own thing. Not a womens night, not in direct opposition to theirs, just a night out talking about a thing you all enjoy. If you start a competing "sports and beer" night at the same time as theirs, people will see it as petty and be forced to choose sides.
Talk with your colleagues and find out what common ground you have. Do you all enjoy underwater basket weaving? Organise a night at the local pool. All enjoy
POPULAR_TV_SHOW, organise a showing, or a coffee break where that is the topic.
Should I take this personally?
Not really, not everyone at work is your friend, but not everyone in real life is your friend either. You can dwell on this, and internalise it, get angry and take that anger to work, then see everything through that lens of anger. Or you can be pro-active, make other events.
I tried all that, no-one came to my events and other people are always being given better opportunities.
Ok, if you've got to this point, you've done everything you can to rule out obliviousness or general camaraderie. The guys who go to these events also seem to be first choice for everything good. Not only that, new guys joining the company get to go, and you are still being sidelined. At the least, its blatant favouritism, at worst blatant sexism.
- Document everything - start keeping a diary. Establishing a pattern is important. Focus on big things, who goes to conferences, who you can tell gets raises (this may be hard), show that its always people who go to the informal 'sports night'. Email to ask to go to things, demand emails back.
- Find allies - Are others not invited, can they do the same?
- Speak to HR - Don't complain about the sports, thats the first mistake, complain about the manager. If you bring it back to the sports night, people will attack it like I did. 'Its harmless', 'its just sports'. Thats when you go, "no, observe this pattern of behaviour". Again, stopping the sports night isn't the issue, its stopping the behaviour. If you get the sports night shut down its a pyrrhic victory, the behaviour just goes underground where it is harder to fight.
- Lawyer up - HR didn't help, and shrugged it off. Good, get it in email, go to a lawyer. You have everything you need for a winning discrimination suit.
- Start a blog - People love stories of women who fought discrimination and won, they also love stories of women who fought discrimination and lost. Either way...
- Clear your schedule - you are going to have a lot of speaking engagements soon.