We pay their salaries yes, but the salaries of US senators are not terribly high (about $175,000) relative to the wealth that many of them live in. Now don't get me wrong, that is a great salary, one I would love to make, but many senators live a lifestyle that exceeds what that salary could bring. Likewise, that salary alone might be tough to pay for a sexual assault payout, as I would expect them to be expensive, granted I don't know much about that.
These senators sometimes come from wealthy families, where they have inherited a good deal of money, a lot of the time they or their spouses might have outside business interests that make them quite a bit of money, or other things like that. The point being, if you commit sexual assault, let your own wealth pay for the lawyers/lawsuit. Sure some of that might come from taxpayer money, but depending on the senator it could vary from very little to much more.
Also, it's not illegal for Members of Congress to engage in insider trading. So yeah, there's also that.