You have some potato chips. ("13 chips" goes on your tally.) You use the toilet twice, flushing both times. ("7 gallons of water" goes on your tally.) You're a bit hot, so I turn up the air conditioning. (I need to break out the calculator, but "+$0.63 for AC" also goes on your tally.) You tell me about a sad movie you watched recently. (This reminds me of a family problem I am having, which is mentally taxing. Since I'll likely need an anxiety pill, I add a fraction of the prescription cost to your tally.) You suggest a walk in the park; we go out. Walking makes me thirsty, so I buy a bottle of water. (But since walking was your idea, I add "1 bottle of water" to your tally.)
Do any of the above come across as petty - or absurd - to you? If so, then that will give you some idea of how someone might feel that splitting social meal expenses down to the penny - and with a written record, to boot - feels transactional. The point is everyone has a threshold where this kind of Venmo'ing behavior goes from innocently equitable to distrustfully penny-pinching.
Venmo'ing someone can be the equivalent of saying, "I don't trust that our relationship will proceed in terms that will average out in such a way that is fair to me." Insisting on Venmo'ing is a lot like insisting that your significant other tell you their email password. If you have to insist this in all cases, maybe rethink your relationship?