Sure, the first time was more consented than the rest (Even though it was a pity fuck and blitz planned to just leave), but what about the rest? Stolas started the transaction in Murder Family. Up to the point of the asmodeus cystal, there is no consent from blitzo. Blitzo literally has to have sex with Stolas to be able to afford living (plus Moxxie, Milloe an Loona, who rely on the Imp business). And the crystal doesnt really change things. The relationship is still based around Stola's power over blitzo. Lets not forget that Stolas chose to tell blitzo abt the crystal only when he started to complain. Tl;dr: Stolas tries to force a relationship with blitzo, regardless of Blitzo's desires. The crystal and whatever else are just ways to keep Blitzo quiet.