we should feel bad about spending money on frivolities rather than donating to charity.

This is standard religious dogma. Secular activists rarely have the gumption to make it part of their pitches.

Interestingly, however, there is some social stigma against donating "too much".

When you take seriously something other people are hypocritical about, it makes them edgy.

most of us are sex-positive activists, and sex-positive activism is arguably an extremely "low priority" type of activism.

Not for me. Keep up the good work :D

Additionally, it is undeniable that someone has to work on the issues I care about, or else who would I donate money to even if I had a lot of it?

Comparative advantage. Compare you being an activist and your donors working (which includes you working a low-value job to donate to yourself) and you working and donating to the marginal activist. Which scenario is superior?

The standard lawyer/secretary example comes to mind- even if the lawyer types much faster, they're better off having their secretary type for them. As an activist, are you a lawyer or a secretary? If gainfully employed, would you be a lawyer or a secretary?