Why is it ridiculous? You just defined sexual assault - sexual touching without consent - why should it matter if it's in or out of a relationship? Your dismissal of a situation which is quite obviously SA based on a relationship whiffs strongly of people who deny that marital rape is a thing. It's not like as soon as you enter a relationship you give blanket consent for your partner to touch you any way they like, anytime. Consent still has to be given each and every time. Heads up, in a court of law it doesn't matter how long you've been in a relationship, if you sexually assault your partner you've still committed a sexual assault.
Consent doesn't have to be verbal and signed via contract. However, it also has to be quite obviously wanted, something the person is comfortable with, which can be shown with either verbal or non-verbal cues. Verbal cues: 'yes', 'that feels good', etc. Non-verbal cues: smiling, pulling the person closer, looking obviously comfortable and relaxed.
Signs of verbal non-consent: 'no', 'I'm not really feeling it', 'I feel a bit tired', 'I feel a bit awkward/uncomfortable'. These are all signs to fucking stop and leave the person alone.
Signs of non-verbal non-consent: looking uncomfortable/anxious, being stiff, pulling away, covering your body so the person can't touch you, pushing them off/away, going silent.
I sincerely doubt that OP is displaying either verbal or non verbal cues of consent when her husband gropes her, as she describes him 'pestering' her, so the touch is clearly unwanted.