Technology

Maybe one could use brainwaves as an input. That'd avoid physical delay. I've got no idea how or if that links to arousal, but I've seen inexpensive, noninvasive sensors before that log it. Using biofeedback off those was trendy in the 1970s or something, had people putting out products.

https://en.wikipedia.org/wiki/Electroencephalography

At least according to this paper, sexual arousal does produce a unique signature:

https://link.springer.com/article/10.1007/s10508-019-01547-3

Neuroelectric Correlates of Human Sexuality: A Review and Meta-Analysis

Taken together, our review shows how neuroelectric methods can consistently differentiate sexual arousal from other emotional states.

If it's primitive enough, probably similar across people, easier to train a meter to measure arousal from EEG data on one set of people that can be used on others.

https://www.sciencedirect.com/science/article/abs/pii/S009130571400032X

There is a remarkable similarity between the cortical EEG changes produced by sexually relevant stimuli in rats and men.

That sounds promising.

There's an open EEG product at two channels without headband for 99 EUR.

https://www.olimex.com/Products/EEG/OpenEEG/EEG-SMT/open-source-hardware

Some more-end-user-oriented headsets exist.

https://imotions.com/blog/learning/product-guides/eeg-headset-prices/

Hmm. Though psychologists have to have wanted to measure sexual arousal for research. You'd think that if EEGs were the best route, they'd have done that, else physical changes.

https://www.sciencedirect.com/science/article/abs/pii/S2050052115301414

Over the past four decades, there has been a growing interest in the psychophysiological measurement of female sexual arousal. A variety of devices and methodologies have been used to quantify and evaluate sexual response with the ultimate goal of increasing our understanding of the process involved with women's sexual response, including physiological mechanisms, as well as psychological, social, and biological correlates. The physiological component of sexual response in women is typically quantified by measuring genital change. Increased blood flow to the genital and pelvic region is a marker of sexual arousal, and a number of instruments have been developed to directly or indirectly capture this change [1]. Although the most popular instrument for assessing female sexual response, the vaginal photoplethysmograph (VPP), measures genital response intravaginally, the majority of other instruments focus on capturing sexual response externally, for example, on the labia or clitoris.

Hmm. That's measuring physical changes, not the brain.

https://en.wikipedia.org/wiki/Vaginal_photoplethysmograph

Vaginal photoplethysmography (VPG, VPP) is a technique using light to measure the amount of blood in the walls of the vagina.

The device that is used is called a vaginal photometer. The device is used to try to obtain an objective measure of a woman's sexual arousal.There is an overall poor correlation (r = 0.26) between women's self-reported levels of desire and their VPG readings.[1]

That doesn't sound like, even concerns about responsiveness in time aside, existing methods for measuring arousal from physical changes in the body are all that great.

As in, maybe measuring the brain is gonna be a better route, if it's practical.