And regarding impurity it is always a sin to put yourself at the occasion in this regard.
As per above, no, not always. There could be a justifying reason, and the reason has to be proportionate to the degree of likelihood and to the degree of gravity. Perhpaps a matrix would help. It's an interplay between 3 factors.
1) gravity of the potential sin
2) likelihood that it would lead to sin
3) degree of necessity
While this isn't 100% sure and some of these could be debated to an extent, it would give you some idea of the dynamic.
Grave Sin + High Likelihood + No Necessity = Grave Sin
Grave Sin + High Likelihood + Moderate Necessity = Venial Sin
Grave Sin + High Likelihood + Grave Necessity = No Sin (remember, just for entering the occasion, not excusing from subsequent sin itself)
Grave Sin + Moderate Likelihood + No Necessity = Grave Sin
Grave Sin + Moderate Likelihood + Moderate Necessity = Venial Sin
Grave Sin + Moderate Likelihood + Grave Necessity = No Sin
Grave Sin + Small Likelihood + No Necessity = Venial Sin
Grave Sin + Small Likelihood + Moderate Necessity = No Sin
Grave Sin + Small Likelihood + Grave Necessity = No Sin
Venial Sin + High Likelihood + No Necessity = Venial Sin
Venial Sin + High Likelihood + Moderate Necessity = Venial Sin
Venial Sin + High Likelihood + Grave Necessity = No Sin
Venial Sin + Moderate Likelihood + No Necessity = Venial Sin
Venial Sin + Moderate Likelihood + Moderate Necessity = No Sin
Venial Sin + Moderate Likelihood + Grave Necessity = No Sin
Venial Sin + Small Likelihood + No Necessity = No Sin
Venial Sin + Small Likelihood + Moderate Necessity = No Sin
Venial Sin + Small Likelihood + Grave Necessit = No Sin
It's something like that, but of course there are more granular degrees than just Small, Moderate, High in terms of Likelihood. Of course, in every-day situations, you just have to go with your best judgment and can't pull out a calculator or protactor to figure this out. This is just an illustration of the interplay between these 3 considerations.