Information reaching Kossyderrickent has it that Ser Criston Cole eating Alicent Hightower pussy in in the room in House of the Dragon S2 sex scene.
For Alicent, there are a few factors. It shouldn’t be overlooked that this is the first example so far of her actually experiencing sexual autonomy: she was pushed into marrying King Viserys and had to do what’s considered her duty; the scenes regarding Larys Strong and Alicent’s feet were about power and information. Alicent is actually in control here, so there’s perhaps a sense of freedom, but it’s telling that Criston Cole is the person she’s chosen to experience it with.
Criston was one of many wedges between Alicent and Rhaenyra when they were younger. The knight sleeping with the princess changed Alicent’s view of her friend, and while they were already growing apart, it helped cement that division. At the same time, Alicent clearly longed for the kind of freedom Rhaenyra had. Having sex with Cole is taking something from her, and getting to have the experience she’d long been denied.
There are several reasons as to why Alicent and Criston have a sexual relationship in House of the Dragon season 2, which goes far beyond desire and two hot people wanting to f**k each other (in fact, there seems to be little suggestion they actually like each other). Instead, it’s reflective of their feelings towards Rhaenyra and towards themselves.