Some of my best friends are sadomasochists but...

The one thing you must never admit when you are a film critic is that a film has turned you on.

Fear, excitement, anger, sadness, joy: all these emotions are okay; part of the aesthetic experience worth describing, analysing and celebrating. But the horn is a definite no-no.

So, when you're sexually stirred by Marilyn Monroe in that silvery breath of a dress in Some Like It Hot, or Brigitte Bardot basking naked in Le Mépris, or Scarlett Johansson luring an unsuspecting Celtic supporter into a vat of alien acid in Under the Skin - which are entirely normal, healthy things for a straight man in his 30s to find sexy - it is considered poor form to let other people know.

Too often, though, this tendency towards discretion goes zooming off in the other direction. Witness the stampede by some writers last year to declare, with a weary and supercilious air, just how unsexy they found the Cannes Palme d'Or winner Blue is the Warmest Colour, an intensely and joyfully sexual film.

There's the whiff of a suggestion that having been turned on by a film is somehow a sign of weakness - that the director has tricked you into liking his work at groin-level, bypassing the more trustworthy organs, the head and heart.

That attitude is dishonest and doesn't reflect particularly well on any critic who perpetuates it without just cause - which I am keen to clarify before writing that I found the newly released trailer for Fifty Shades of Grey about as arousing as week-old meat loaf.

This two-minute clip gives us our first teasing glimpse of Sam Taylor-Johnson's forthcoming film adaptation of the incredibly popular EL James novel, in which Anastasia Steele, a student played here by Dakota Johnson, strikes up an unconventional relationship with Christian Grey (Jamie Dornan), a successful young entrepreneur with a penchant for sadomasochism.

Anastasia is the vulnerable ingenue, which we know because she has a fringe and wears a blouse three sizes too big for her.

Meanwhile, Christian drapes himself broodingly across a grand piano. He removes his shirt a few times, then there is a timid glimpse of ropes and riding crops.

The clothes, the decor, the shiny piano, the helicopter: is it just me, or does it all look a bit like the "win Simon Cowell's lifestyle" clips they run as competitions on The X Factor?

If this gets you going (and the million-strong sales suggest it must be doing something for someone) then far be it from me to judge, although having now watched the thing five times over, I'm still not entirely sure which are supposed to be the sexy bits.