Take it from an older lady who has, ahem, been around the block. You're very sweet to love him and his family and all that.
He doesn't deserve you. He will never act well. OR MAYBE he will when someone ties him down to be married, in, um...I don't know...15-20 years. But he'll probably still act badly then.
I know that sounds simplistic and hard to swallow, but it is true, you will see.
Ideally, if you're brave and busy: End it. Tell him, "You know, I think it's great you're so full of life and attracted to fabulous women, I totally understand why they find you appealing and vice versa, but this will never be OK with me. Take care, have a nice life. Maybe we'll talk after Australia. I'll let you call me if you want."
Girlfriend, my motherly advice to you is what it would be to my daughter. I used to pay my own way in NYC, and that makes you a SUPER TROOPER WAY above the cut. You're young, you're beautiful, you're healthy, you're a good person who knows how to treat people with respect. You deserve the same. Don't stand for less. There are MANY more fish in the sea. You do not need hurt and jealousy creeping in to taint this brief awesome phase of your life. Don't shut doors while this guy takes up your spare time.
The trouble with being a 21 year old yoga person (been one of those myself back in the day too) is that you're only one of tons of other young beautiful people. All with no attachments, all with lovely sparkly personalities, and elevated zen mentalities, all with hot bods. Men that age are rarely able to pick out what is really special about YOU in PARTICULAR, you're just another awesome, smart, independent, fun, pretty chick. It sucks to find out the guy you thought you knew so well, didn't really hold you above the crowd the way you did him.
I've been with men like that. REALLY lovable charming ones too. It sucks. It only gets worse. Never better. Even the ones who seemed to quit ogling other chicks, ended up having well hidden disloyal traits. It's insecurity. It's extremely bad behavior. Nice men NEVER do that. Of all my friend with AMAZING husbands: none of them are flirtatious. All my girlfriends who dated or married flirtatious guys: It grossed out their friends, it ended badly. It's not cute.
Be thankful for the good times and move along. You nailed it on the head, respectfully dismissing him NOW, and telling him calmly and sweetly why, will be the ultimate "keeping of your dignity."
Wish someone gave me that talk.
Have a blast in Australia!