Since Vulcans are emotionless, how do they get into the mood for sex?
How do they solve the problem of reproduction when sex is not possible?
Trek writer Dorothy C. Fontana stated in an interview (for Captains' Logs: The Unauthorized Complete Trek Voyages) that Vulcans are both capable and willing to have sex outside of this period, presumably for the purposes of procreation and also to cement emotional closeness:
"Vulcans mate normally any time they want to. However, every seven years you do the ritual, the ceremony, the whole thing. The biological urge. You must, but any other time is any other emotion—humanoid emotion—when you're in love. When you want to, you know when the urge is there, you do it. This every-seven-years business was taken too literally by too many people who don't stop and understand. We didn't mean it only every seven years. I mean, every seven years would be a little bad, and it would not explain the Vulcans of many different ages which are not seven years apart".
The other misconception is that Vulcans are emotionless. Note that every Vulcan we see in the show is struggling to keep their (ultra-strong) emotions in check. When this mental control fails, they generally go berserk.
Vulcans have emotions, very strong and intensely felt; this is repeatedly affirmed throughout the various series, most pointedly in the TNG episode "Sarek". And Vulcans have sex for recreational purposes as well as procreative (T'Pol and Trip got it on a few times in ENT IIRC).
Vulcans used to be savage and warlike, far worse than humans ever were, such that they nearly wiped themselves out without the need for WMDs; Surak recognized that Vulcans would not survive unless they found a way to supress most emotional reactions, hence the philosophy of pure logic (again, multiple episodes - "The Savage Curtain", "All Our Yesterdays" from TOS are the ones that come to mind immediately).
Secondly, arousal and emotional response are not necessarily the same thing. Biological imperatives will win out over philosophy. Pon Farr is the price Vulcans pay for keeping their emotions so tightly bottled up.
Every seven years they go mad with emotion and must return to their birthplace so they can mate. See "Amok Time", Original Star Trek series.
Vulcans have emotions, pretty volatile ones, but since the times of Surak they try (with more or less success individually) to control their emotional responses by using logic. Only a master of the Kolinaar is without emotions, but they are a minority. Vulcans are capable to have sex and procreate like any other biological being. The "Pon Farr" is a distinctive feature of the species, with chemical imbalances in the brain that will shatter the logical control. Either sex or fight is used to purge the chemicals out of the body and return to a state of equibrilium.
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