He means that when you follow a singles path, either of them, there's no more interaction between them. Maybe between you and them one as lover the other as friend (I have no clue if it's even possible to keep the friendship) but they will be at the best chance, friends.
That means, no threesomes. No lesbian interaction. Maybe not even talking, much less touching.
No, this isn't a "Have the Cake and eat it too". You keep Alex and refuse Isa, or, you dump Alex to pursuit Isa, or, you keep BOTH. If you keep both, they will be in a relationship with each other TOO.
So, choose your poison.
Peace