The gemara in Pesachim [72] says some SHOCKING things. The gemara teaches that there is something called "simchas onah". What is "simcha-dike" about onah? It is soooooo not holy??!! It is SHMUTZI!! GOYISH!!
The answer is that it is a MITZVA MIN HA-TORAH. Just because goyim do it doesn't make it impure. They do it in an impure way and flood us with images day and night including the woman reporting the 6 o'clock news where she got the job not because of her great understanding of current events but because of her physical [I am avoiding the common word] appeal. Society makes it dirty but for us it is a mitzva.
Rava says that even if it is not officially a mitzva of onah - there is still a mitzva to be with one's spouse.
The Rav ztz"l has a teshuva in the Ezras Kohen where he references the gemara [which one should see inside], explains why it is permitted even when one's wife can't become pregnant and defines the mitzva:
This is a new [for some] vista in hashkafa. What does Hashem want from us in this world??
