OBVIOUS reason #1: Why do guys want RAINBOWS on their... well on any body part for that matter. Rainbows are connected with being gay. So why a straight guy would want a rainbow colored dick is beyond me.
OBVIOUS reason #2: Does it not disturb any of you guys out there that the girl who is putting her "mark" on you just had her mouth on another's guy's dick? Imagine this: precum from another guy being transferred onto your dick from this girl's mouth.
OBVIOUS reason #3: Wouldn't you much rather a girl give you a full on blowjob than to give you her "mark"?