Whether a privacy like sex should be open to all should be scrutinized for everyone's opinion. But I personally think that it is not even understandable to transform a privacy like sex into a sport and open it to everyone.
If a casino or gambling site were to convert a private activity into gambling based on sex, I would not bet on this forbidden act because there are religious restrictions within which my religion has never legalized open sex.
Definitely, religious people won't be betting on this one. As you have mentioned, it's a private thing being done by partners and with that, the intimacy should be kept privately. And for the religious people, aside from being vulgar and liberated that should be avoided. Gambling itself isn't taught in religious gatherings and the members are discouraged to engaged into such. But you know what's with the reality nowadays, we're not perfect and you'd see religious people engaged into activities that you cannot even imagine.
In Islam gambling is a no-go area, it is forbidden because it is a grievous sin. But in some religions, gambling is not mentioned in their Holy Books. However, some religious leaders always discourage people from engaging in gambling mainly because of addiction. Gambling is done to raise funds for religious events and gatherings. It is generally seen as a means to raise funds for religious purposes but any event that gives you the privilege to put in funds to get more is gambling. For me, responsible gambling should be our watchword but if you cannot control yourself, stop gambling.