A few examples:

Example A: I am with some friends at the bar. I see a girl, walk up to her start chatting eventually she ends talking with all of us. Exchange numbers. Scarcely sending some text messages(teasing a little bit) and text messages change to rather sexual. Then it stops.
Later I see a mate of mine from the same night at the bar, apparently he exchanged his number too and been texting her ever since.

Example B: Another night, another girl, been dancing with her. I take a break and just go chat a bit with mate, then another friend (same as ex. A) just goes towards that girl and starts dancing with her.

I'm all up for a fair game but I am not getting the impression anymore that it's a fair game. I am feeling that there are structural problems and that I am doing all the 'meet and greeting' and some other people are just ratting in on what I can pull. And it is happening on a more structural level, it's not seperated cases anymore. It happens often.

What can I do against this? I don't intend to go out anymore with the person in ex. A & B. But I am trying to arm against this happening in the future. It feels to me that some others don't respect me.

Don't bring douchebag friends around.

Yeah true, but I get the feeling that for everyone of those I 'cut' out of my friends circle, there is another waiting to be the same.

Sounds like they're just thriving off of your game, instead of establishing their own.

Just keep a distance from them in clubs when you're trying to do your thing. Or just let them know that you want a certain person and for them to back off.

I'm not PUA, just a regular person and to me this sounds like bs that I'd be pissed about.

You don't have to go out with friends. Hanging out with friends and gaming often doesn't work out. Instead just go out to a bar or wherever you game and fly solo. Then no one can ride on your game.