Some people I think confuse the two - sex and fetish. Or, perhaps more specifically, they treat them the same, whereas I think they require different approaches when trying to be a good partner.
Let's tackle them individually. (Examples given below refer to a partnership of two, but are all applicable to larger dining parties. These are all the opinions of me and me only. Your mileage may vary. Offer void in many states.)
Should you have sex with your partner when you are not into it/feeling inspired? The on-paper answer is no. In my world, sex is good when both partners want it. I am only happy in my sex when the other person is perceivable into it - either visually, audibly, or participatory. Now, dry spells happen. Is it nice to give the other a quick "wank," to use a horrible turn of phrase, once in a while during these dry spells? Yes, I suppose it might. But not necessarily required. The trick here is to recognize that the lack of sex can sometimes breed confusion and doubt. Communication is the key here.
Fetishes. Also read as: Really specific interests/fantasies.
Here is a tough one. Let's refer to the GGG principle again. According to that philosophy, you are good partner if you show an acceptance on a participatory level in the specific sexual interests of your partner. OK, great. Now, does that mean you have to be actually into it? I'd say no. Example:
I have a foot fetish. I enjoy incorporating that into my sex life, nay, I'd say I have to have it incorporated in my sex life on some level. If I go without it, I am very sad. Now, am I to expect my partner(s) to be into it as much as I am? Absolutely not! Just because I am into feet does not mean my partner should/will be/has to be turned on by my ministrations on their feet. I do not need that to get off. What I need is acceptance, and the willingness to allow me my fun once in a while. I need to feel normal and valid despite having these wants, and I'd like to get them fulfilled once in a while. Not daily, not even weekly. But every now and then, yeah.
If I find you are into my particular brand of perversion, fabulous. We'll indulge ourselves until we drop. But, I don't necessarily need that.
Because as a fetishist, I have a rational understanding that not everyone is going to share my tastes. In fact, I know that some of my tastes might be shared by only one in a million, or even never. So, with a clear conscious, I cannot expect my partner to be into it. That's just not logical.
So when indulging, you do not need to be turned on. You could be doing it out of friendliness, or maybe curiosity/fascination. Just keep it positive, and we're all good. And again, just once in a while.
And I'll do the same for you. Because I also want to be a GGG partner.
There are some tastes that are so out there, they might never happen. I'm OK with that. Maybe one day I'll find someone to partake with. Maybe not. That's OK too. I am OK with that because I understand how out there some of my tastes are. Communication - always great.
In the kinky universe, there is a lot of stress about this. What are otherwise wonderful relationships can become stressed to the point of breakage just because of fetishes/fantasies. I think there is a way for people to meet the middle, and have everyone walk away content. Some people place expectations on either themselves or others that because they are into something, the other partner should be as well. Absolutely not. And we would do well to understand that, allow ourselves to accept that, and then find a middle ground for mutual fulfillment.
Disclaimer - these opinions assume partners are on an equal level in terms of day-to-day power exchange. When it comes to continual power exchange relationships, all of the above could be thrown out the window. I am merely writing what is true in my life, which is all I really can write about.
I should add that when in a D/s context, all of the above still applies in my life. But I know, not others.