I grew up thinking the shiva linga was a combination of penis entering vagina and I was fine by it because it represented source of all creation. I think there are new schools of thoughts on it too but I would not disregard the penis/vagina school of thought.

There is nothing wrong with worshipping the act of coitus. We are all results of that union and if we try to regard it as bad then ironically you are not respecting your roots :)