Cameron Diaz as Mary Jensen in There's Something About Mary (1998)
As Clarice Starling will tell you, there's nothing funny about having ejaculate in your hair.
Having said that, Cameron Diaz famously played the idea for laughs in the Farrelly brothers' comedy There's Something About Mary (1998), so I suppose it's contingent upon a number of factors, such as whose semen it is and how it got there.
So, it might be fun; it might even be pleasurable. But it might just be disgusting and involve an act of sexual assault. It depends.
Interestingly, while some tricophiles are happy just to sit and comb their lover's hair; others need to come on the object of their desire, which is all fine providing they have consent (and maybe the courtesy to provide a luxury shampoo in return: quid pro quo, as Hannibal Lecter likes to say).
Finally, it's worth noting that whilst having semen in your hair won't cause any harm, there is little evidence to suggest it will do any good; claims that it is a natural conditioner, full of proteins and vitamins, that will leave your hair super soft and shiny or promote growth, are mostly nonsense.
Not that this has stopped some wealthy Californian women who can afford treatments containing bull semen from popping along to the salon and demanding a dollop of the latter be mixed into their honey, avocado, and argan oil hair recipe [1].
Anya, an editor at the Huffington Post, volunteers to have a bull semen shampoo and set.
'It smelled pretty nice - kind of sweet - and had a smooth texture.' [2]
Notes
[1] See Meagan Morris, 'Bull Semen Hair Treatments Are a Thing', Cosmopolitan (6 Feb 2013): click here.
[2] See Dana Oliver, 'LA Salon Cooks Up Bull Testicle Hair Treatment And We Tried It!', in the UK edition of the Huffington Post (05 Feb 2013): click here.
Anya concludes that there was very little difference in her hair texture post-treament and that it's probably best to stop wasting time, money and energy on expensive beauty treatments with little or no evidence to show they work.