I know, I'm already at loyalty 70 thanks to the "Just relax" option, but the problem is that Betsy has corruption equal to 60, and to end the masturbation scene she "needs to be aroused" meaning that the corruption level must be greater or equal to 70
Yes, indeed ... there is a rather complicated system with thresholds, which definitely needs to be simplified or painted in more detail.
In general, here is the exact information - if Betsy has a corruption of 60 and higher, then when she jerks off to Gaston under the table, then her corruption can be pumped up to 70.