50 yr old woman on dating scene and wonders why boyfriend wants to hang out with two young woman?

I'm a 51yr old woman. I'm just new on the dating scene of late. I hooked up with a man about 3 weeks ago and we've met every day since. He is 54 and a retired plumber so hes lots of spare time. We like to hang out in the pub but now somethings bothering me and i'm wondering could you shed some light on it. One night 10 days ago we were in the bar when a girl approached us and started chatting. Shes only 27 and has a very sensual figure..id say she dresses trampy but maybe i'm old fashioned. She stayed with us th whole evening and we had a good time. However, my boyfriend has started saying strange stuff like he thinks shes 'open minded' and would be willing to 'experiment'. Shes come out and told us shes done s. I just hope that is not what shes looking for as shes in the bar every single time were are there now and ends up joining us. I make excuses and go home early when she arrives. I dont know how long she stays wtih my boyfriend. Recently another really tarty looking younger girls is with her, wearing hotpants and hooker boots that skim her bum cheeks. Every man in the bar drools and they all try it on with her. Whats going on here?