Amatree has just trotted out the stock response, complaining that it is abusive, lazy parenting etc.
Smacking is legal, it is not abusive, or lazy parenting, although it can be, it is no more abusive or lazy parenting than shouting at a child or not disciplining them at all ( personally, I consider those more abusive)
ANY form of discipline need a calm instruction, followed by a warning, followed by the warned of, known about and understood consequence. The consequence needs to be a deterrent, and for a small child, a light slap is a perfectly acceptable deterrent.
You will get thousands on hear shreiking and screaming at that, without stopping to consider how much more humane a light slap is compared to shouting at a child, or neglecting their discipline altogether, so be prepared for that. They don't seem to get that millenia of human biology and neurological evolution are not going to be over turned by 10 years political correctness!
However, the deterrent is only going to work if it is a last resort the child wants to avoid, and is given the time to consider, and choose to avoid.
Equally, a different deterrent is just as acceptable, if it is effective, so it isn't the deterrent itself, but the way it is applied that is important.
The way your DP is applying the deterrent is ineffective - even if you do agree to a smack - and if you are so dead against it, he should agree to something else anyway, time out, or something.