Syx doesn't really know how to handle the thought of marriage especially since the proposal was so emotional and... he just has problem believing that someone would want him around like that. But he's also incredibly happy and relieved that Decay wants him so much. After all he loves him super dearly and would do nothing more happily than to marry him.
I imagine this scene ends with many hours of very gentle loving in the bedroom... XD What a valentines day!!!