The western blot looks for all 16 antibody proteins and four must be present to be called positive while the IgG test looks for a single antibody protein that is different between HSV 1 and HSV 2. You can see that the blot is way more thorough and the chance for a false positive is zero.
During sex, both things happen a bit – fluids and skin to skin contact. But the reason condoms are so effective at reducing transmission is that they prevent both. It’s a bit hard to separate completely