Examples of applying natural law

Natural laws application to modern ethical issues is its relevance to sexual ethics

Aquinas believed one of Gods purposes for the human race was to 'be fruitful and increase in number' Therefore aquinas claimed one of the primary precepts was to reproduce 

In order to follow this principle every sexual act must be open to the possibility of having a child

Any sexual act which does not open to the fact of having children is immoral according to natural law. Therefor the use of contraception is immoral.

In contrast

If roman catholics priests do not produce offspring is this not also opposed to natural law? Aquinas was a priest and remained celibate he argued that the majority of the population should focus on reproduction.