Just from taking her skirt off, his pants were already soaked through with precum.
Literal
Skirt [obj-を] remove only-[-だけ] [with-で], his pants [topic-は] already precum [with-で] soaking.
だけで 'just by / merely by X' is the frame for 'X alone was enough to cause Y' — used to dramatize how little stimulus produced a big effect. 我慢汁 (literally 'enduring liquid') is vulgar slang for precum. ビショビショ is a state mimetic for 'soaked / drenched / wet through' — typically used for clothing, hair, or ground after rain. The whole sentence is clearly adult-content register.