You can tell Halsin you're monogamous and don't share his nature. It's just that it doesn't result in him changing his entire personality for the player. It instead results in both of them realizing they are fundamentally incompatible people and the relationship progressing no further. Which is how it should work. Halsin is polyamorous and does not want a monogamous relationship. The player in this case is monogamous and does not want a polyamorous relationship. There is no point in progressing further.