If you are buying SexCoins directly you can't earn SexCoins in the game. However, all purchased coins and items will stay forever (as long as thriXXX offers the service).
If you subscribed for a recurring membership you earn SexCoins when you play the game and bring the models to orgasm. (Note: after cancellation and expiration of a recurring membership all coins and items will be removed and your account will be downgraded to a free version again)
Depending on the chosen mode you earn different amounts of SexCoins.
The StoryMode is the mode allowing for the highest amount of SexCoins to be earned. In StoryMode you get SexCoins for successfully completing a story as well as for the quality of the sex, or rather the orgasm(s) during the sex.
Note: The calculation and allocation of SexCoins takes place AFTER bringing a model to orgasm. If you cancel a game before this, you won't get the SexCoins you would have earned if completing the story. There is also a 24h limit of how much SexCoins you can earn.