Some good points are made here.
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there are multiple instances in this game where Dylan and Sam are together when earlier feelings are expressed and physical contact with Sophia take place. Why should the bikini event be any different; one might argue, this is the first time Dylan is given a choice to have Sam with him or not; when he say's yes it could be that he wants to share her with Sam as well as Sam is obsessed and keeps saying how hot she is and all that, earlier in the game he was even jealous / Maybe it;s a chance for Dylan to make Sam Jealous and prove or show to Sam how he has been able to change Sophia's mind to do things for him and even wear bikini and let him touch and all that (To show who is the boss now kinda attitude) ; so technically yes, I guess you can say it's a thin line towards cuckoldry and voyeurism combined but what if Dylan has no intention to share Sophia with Sam , and only wants to tease Sam a little to show his progress with Sophia. The question you should be asking is will Dylan allowing Sam on the bikini event without points open up scenes with Dylan and Amber in the near future (Sam and Dylan making a deal of some sort).