In Spider Man Reign, Peter Parker passes radioactive semen into Mary Jane, which caused Mary Jane to get cancer and die. To my mind, this qualifies as a sexually transmitted disease.
"Oh God, I’m sorry! The doctors didn’t understand how it happened! How you had been poisoned by radioactivity! How your body slowly became riddled with cancer! I did. I was… I am filled with radioactive blood. And not just blood. Every fluid. Touching me… loving me… Loving me killed you!"
Are there any other Marvel characters that have been shown to have sexually transmitted infections?