Does liking the idea of something mean you'll probably like doing it in real life - or in other words, do you think we should act on our fantasies?
Short answer, no.
Having a fantasy about something certainly doesn't mean that you would enjoy that thing in reality. Fantasies do not necessarily match up with real life sexual preferences. For many people fantasies are strictly fantasy. And part of the enjoyment is in knowing that they are just that. Sometimes the erotic imagination travels to places that are arousing purely because of how subversive or deviant or depraved they are. In fantasy anything and everything is possible and nothing is 'wrong', however, this is not at all the case in real life for a number of reasons. For example, you might fantasise about having sex with your boyfriends/girlfriends dad/mum, but the prospect of it actually occurring in real life is rather more weird, creepy and stressful than sexually arousing.
BUT I don't like short answers. So long answer is: sometimes yes and sometimes just a little. Sometimes sexual fantasies can be a good indicator of true sexual desire
Some sexual fantasies can not be acted upon because they are literally impossible (E.g. they may involve mythical creatures or be highly illegal), but many times sexual fantasies can be made a reality, and it's important to be really cautious in doing so. Often fantasies can be incorporated in watered down ways, such as through role-playing, rather than fully realised.
I'm going to use two common examples in order to best illustrate the variation in the fantasy versus reality conundrum. But you have to always keep in mind that fantasy and reality will never completely align because, well... life.
A clear example is that of rape fantasy. 'Forced sex' fantasies are actually very common amongst women, but OF COURSE NOBODY wants to be raped or sexually assaulted in reality. So in order to fulfil a rape fantasy some people will engage in 'rape' role-play, in which forced sex is simulated. It's never actually rape because the act is fully consensual and has been previously discussed and agreed upon by all those involved. So the fantasy is acted out in a fantasy scenario, NOT in reality. In this type of role-play a pre-arranged "safeword" is a really good way to ensure that ongoing consent is obtained and all parties are happy throughout.
Although a common one, rape fantasy is a fairly extreme example of where fantasy and reality are completely separate. There are milder fantasies that absolutely could translate to real life enjoyment. So in some cases, you absolutely can and should go ahead and make your dreams come true. The best way to do so is by using a graduated approach, so if at any point you realise that this fantasy is best left a fantasy you can do so without having caused too much distress.
My second example, and probably one of the most commonly acted out fantasy is that of a THREESOME. MMF or FFM threesomes are top of the list fantasies for many people. And while some people love threesomes, many other people find that the reality does not at all match up to the fantasy.
I think this is a common reaction (especially with threesome fantasies) because in your head other people are part of your imagination and you are in control of every outcome of the fantasy scenario. In reality other people are human beings and you don't have control over how they react and engage with your fantasy.
Quite often the worst thing you can do is to immediately try and recreate your ideal fantasy scenario. A graduated approach can be used in much the same way for most fantasies. You should move from simply talking about the fantasy, through to dirty talk and role-playing, through to acting on the fantasy in slowly and gradually more involved ways. An example for a heterosexual couple interested in exploring FFM threesomes is a visit to a strip club and a purchase of a couples dance as a way to lightly test boundaries together.
Whatever the fantasy and approach, at each stage you should do a little 'reality check' to see how your real life experiences are aligning with your fantasy expectations before continuing to the next stage, deciding to stay on the stage your at, moving back a stage, or aborting the mission altogether.
Bivona, J., & Critelli, J. (2009). The nature of women's rape fantasies: An analysis of prevalence, frequency, and contents.
Joyal, C. C., Cossette, A., & Lapierre, V. (2015). What exactly is an unusual sexual fantasy?(2), 328-340.