I thought Evil Aliens had the most heinous alien impregnation scene... until I saw 1981's Inseminoid, a low-rent Alien clone. And yes, this alien has a bong for a penis. Clip is NSFW.

In Inseminoid, a team of archeologists land on an alien planet, and discover some weird crystals. Which make them go nuts, or something. They all start dying, in totally ludicrous ways. Then the ship's doctor, for some reason, injects Sandy with something and gets the bug-eyed alien to inseminate her. Afterwards, the now-pregnant Sandy goes around killing the rest of her shipmates, before giving birth to aliens. (Weirdly, the ship's doctor, and the alien that knocks Sandy up, both disappear from the film after the half-hour mark. You really only glimpse the daddy alien for about 20 seconds of the film, total.)

Inseminoid may actually be the worst] science fiction film ever made. Well, maybe not. But it's up there. Interestingly, it came out in the same year as Galaxy Of Terror, another entry in the disgusting pervy alien genre. [IMDB]