Spike has a harem, in Twilight Sparkle’s basement.

These lost mares are wayward and has not been asked for, in what Spike deems a sufficient period of time.

Knowing ponies and mares from Rarity, Spike knows the tricks required; to maintain his domesticated harem docile and manageable.

What could possibly prevent Spike from  having just a little bit of fun on the side?  It isn’t as if the mares would complain.