I haven't really noticed that-but I think the reason why that is, is because Sesshoumaru is a mature character-mentally and emotionally. His mature nature can bring out Kagome's maturity, and they can go further in their relationship(marriage, children etc) because Sesshoumaru can provide the stability I could see Kagome wanting for those things.
Where as Inuyasha is very immature mentally and emotionally. Maturity cannot come over night, or even in a few years, I think it would take at minimum five years to finally 'grow up' or mature mentally and be emotionally and mentally ready for the major events of marriage and having/raising children.
If that made any sense at all?