Honesty isn't always beneficial
The pain of being cheated on is only felt when one discovers they were cheated on. Similarly, the agony of being in a loveless relationship is only felt when one side really get it hit home that they're in one and that this needs to change. It's one's right to be ignorant of their relationship's passion and to stay together for rational reasons, such as the emotional wellbeing of their offspring.
Love isn't at all the only thing to measure and weigh up for a couple
Having a love meter would revolutionise how couples measure their success and approach a lack of passion. Instead of valuing trust between them, simply enjoying one another's company in an asexual manner (or lustful manner if its the opposite scenario), they will learn to only value the degree of passionate affection felt on an emotional level for each other. Let's say one partner is depressed or feeling less love for a variety of reasons that aren't to do with valuing their partner and the relationships less, this would inspire people leaving partners when the relationship should be fixed, rather than fixing it and helping their partners out of the passionless rut they have gotten themselves into. This would not even help couple's therapy because it would completely violate the part of therapy which is encouraging one to open up and consensually reveal to the other how they feel, instead it would become a blackmail scenario where it's 'what have you got to hide?' Imagine couples where one partner highly values personal privacy and loathes surveillance but the other does not. This would become a very toxic situation where one would keep refusing to be measured because they don't want that invasion of privacy and the other calls them disloyal and loveless due to that... It's a terrible idea.
Why would this even work?
I am not asking 'how' but 'why' it would work. Even if you could measure the degree to which one feels intensely emotional in a positive manner around another, what about the anxiety they feel about being measured by the love meter? How can the love meter distinguish that from the anxiety felt regarding the partner? How about the fact that maybe someone is cheating on their partner and is a sociopath who enjoys it? Wouldn't they feel very passionate around their partner?
I am not exactly dissecting the scientific specifics here, I am saying that the core reasoning behind more vs less love felt at any given moment isn't always about the success of a relationship or genuine devotion to one's partner.
There is a reason that polygraph tests can be tricked:
What about open relationships?
Open relationships can be healthy, some would argue the world would be a better place if we weren't pushed into monogamy and if neither partner in a relationship felt a deep pressure to make the other love them, instead letting things naturally flow in an anarchic manner where each is with the other while their passion lasts, goes to another and comes back in a polyamorous manner.
A love meter would only seek to incite conflict and toxicity in an otherwise healthy arrangement where neither needs to envy the other's partners.