I imagine it's more semi-noncon to consensual noncon there. Sandstorm was having wet dreams of Spottedleaf, which is what got her attention. I imagine as Fire got older, he lost his infatuation and more just appreciates her presence and now Spots is bored af.
Anyway, Spots takes note and starts small, affecting Sandstorm's dreams bit by bit. The nice thing is she's not bound by the laws of physics, her being a spirit and this being a dream and all, so slowly but surely things get more extreme. Sand just thinks she's having more intense dreams than usual. Then Spots realizes she's actually getting really into this and stops completely as she sorts out her feelings.

She comes back and actually goes and talks to Sand like "hey I've been stalking your dreams and twas me who made you gush in your nest every night. I can keep doing that if that's cool." Sand is obviously embarrassed. But after she gets over it she's like "what else can you do?" The rest is history.

Whatever weirdass, kinky sexual fantasy you've had, they've probably done it. (also bdsm with Spots as a dominatrix is one of their favorite scenarios.)