Sven isn't polyamorous, he's a slut. There's a difference.
Sven's not a slut.
Sven's a guy.
Sven's a regular guy.
He happens to be an attractive guy, but he's still just a guy.
This means that he has absolutely no defense mechanisms.
When a girl turns 11-13, she grows tits. From that day forth she is actively accosted by males who want to do the bone-diddly with her. She learns from an early age how to say, "Get the friggin heck-fire away from me, you gosh darn creepazoids!" without actually saying it.
When a guy turns 11-13, he grows pubic hair, gets incredibly horny/clumsy and embarrasses the living shit out of himself every day for about 10 years. (give or take) He doesn't get hit on by girls, he doesn't get regularly/aggressively propositioned by every girl he meets.
Women learn to say "No.", to build walls, to defend themselves against men who find them attractive.
Men don't.
Sven has never before been in a situation where he should say "No." He's never before had a woman make sexual advances where he had a damn good reason to say, "No." Saying "No" gracefully takes skill, it takes practice. I mean, a girl tries to kiss you. A beautiful girl. A beautiful, talented girl you respect.... but you're in a relationship. You, with the five serious relationships under your belt. You might be able to say, with regret all over your face, "I... I can't. No! It's not you! Shit! Don't cry! Shit! No! I... well, there's this girl, and... I'm sorry!"
Has Sven ever had a breakup? Or has it all just been mutual "Well, that was fun"?
We've seen him try to avoid sex twice. Each time he had to resort to cheap tricks and attempts to get other people to do his dirty work. Because Sven is immature. He's a 30 year old virgin when it comes to relationships. He's doing the right thing when it comes to telling the truth.
But he has never learned how to say "No." Most guys don't; Sven's just more ignorant than most.