I like that George sent that small but important message to Caroline; giving her some hope after the way he verbally eviscerated her. I know that at it's crux George is more upset about the lying, the deceit and that it happened whilst he was onshore than the actual encounter and normally i'm 110% team Granger. Having said that, I can see why Caroline resents the sexual freedom George has and she doesn't. This is manifesting through her sexual desire to be with men George has been with. Instead of a discreet sexual encounter often, which i'm sure she would be more than capable of having given her connections. I think she has just as much a desire to have sex as George and wants that freedom but went about it in the entirely wrong way. I can't remember when or how she gave her blessing for Granger to sleep with men but clearly while she's "alright" with it in principle. The fact though she has to remain faithful irks her. George can gallivant with anyone nearly and does (not that we're complaining) whilst George (it must feel for her) wants her to remain chaste. This may be because George on some level views her as does society as property, his, but I don't think Caroline's ever really liked the idea of being owned.
I know this is due to the risk of gossip, patriarchy and pregnancy (again). Having said that there must be a middle ground where Caroline can have her fun too. I know it's George's story but the idea that women don't enjoy sex or want it on the whole as much as men I've always thought was woefully inaccurate. At least buy her a vibrator, they've been in France since 1734 and called the "Tremoussiour" (I know Wikipedia cardinal sin of referencing but it's more of a jest). It really does speak to the whole time period it's set in of course though; women as irrational, not thinking of the good of the family, wanton and craven to be locked away in a monastery or castle. The way she has treated Granger was wrong but given the inequalities we're dealing with, interpersonally and on a larger scale I understand it.