Regarding Sandy and Nicole, the mc had just that
very same afternoon told Jill he was falling for her, and then a few hours later two woman want him to come home for a threesome. It's justified he felt a twinge on his conscience at that point (or he was still thinking about what a lovely afternoon he had) and decided against it.
But with Riona & Cammy, he doesn't get to fuck them until a week later at the party. So he went for it. Plus, fucking a girl in her room at a party (which may or may not end up in a threesome) is not the same things as going home from a bar with two women. One is really spur of the moment, the other is a little more premeditated. Not that a girlfriend would accept that differentiation.
Not sure if you're looking at the Sandy/Nicole vs Riona/Cammy thing as simply, "he did one threesome, why not the other? Doesn't make sense?!" But the timing makes a lot of sense.