OK, I deleted the post, since I noticed I was necroposting to a guy who posted back in January, but I see you people still don't get it, so fuck this, I'll write this God damn essay again.
Allow me to introduce you to this thing, called "Oxymoron"
Stemming from Greek Oksys (sharp) and Moros (dull), it's a stylistic phrase, where you join together two contradictory expressions, e.g. "dark flame", "glorious humiliation" or "speedy as a snail".
It can be used for many reasons, but mostly it is a way to bring attention to the thing being described, as well as to emphasize its unnatural or conflicting traits.
In this instance, "spicy" has nothing to do with "spice", but rather with the adjective, meaning "hot", "irritating to the mouth and throat", but also more modern, slang meaning of "kinky", "sexual in a creative way, that is often not commonly accepted within society". It is contrasted with "sugar", which is sweet, but also used to designate a very vanilla-like, wholesome relationship, where two people are madly in love with each other, to the point of idealizing their partner.
The author is most likely making a play on words here, trying to at the same time emphasize both the romantic and sexual nature of the relationship between the two main characters. "These two will be madly in love with each other, but will also have quite a bit of sexual fun, spicing up their relationship".
Everyone knows sugar isn't spicy. That's the whole point.