Doing some toilet-bowl Googling, I found that the word “come” has been used as a euphemism for orgasm or ejaculation since at least the 17th century. It wasn’t until the latter part of the 20th century that the alternative spelling “cum” became common, specifically to remove the ambiguity between the sexual meaning and the ordinary verb come. So you were absolutely right to prefer the spelling “cum” for that reason. The spelling was then further popularized through the rise of pornography and erotic media in Western culture.