Clegg's wife was Pearl's sister.
Pearl clearly looked out for him and treated him with kindness — she sometimes bought milk for him, and in one episode she sweetly wished him happy birthday which was out of keeping with her usually stern character. A possible reason for this would be that he was her brother-in-law, and she worried about him because her sister Edith, who died about a year before the show started, was no longer around to look after him (it would be entirely in keeping for a woman like Pearl to think that a widowed man would be somewhat incapable of looking after himself). When Compo died, she voiced her concern about how Clegg would miss him, having previously seen how upset he was when someone close to him died (whenever Edith was brought up, it was clear that Clegg remembered her fondly).- Perhaps, deep down, she regretted that Clegg married Edith instead of her, while she got stuck with Howard. Certainly, she knew — as did everyone else — that Edith made a better choice of husband than she did.
- This would also explain why Clegg was so tolerant of Howard; they were clearly not friends, but he couldn't tell him where to go because he was family, and being rude to his brother-in-law was not something an all-round nice guy like Clegg could bring himself to do.
Howard and Marina never actually committed adultery.
He was way more interested in coming up with the many and various Zany Schemes to facilitate their relationship than he was with actually consummating it. A classic example is the time he claimed to have taken up birdwatching as an excuse to meet up with her at secluded locations, only to end up spending his time at said locations actually birdwatching.- Marina for her part was only too happy when other men showed her affection — even Compo — and obviously had a thing for Clegg, but since the former was more interested in Nora Batty and the latter was obviously not interested in pursuing a romantic relationship with anyone, she had to tolerate being with Howard.
- Pearl was of course well aware of this, and while obviously not happy about the fact that her husband (whom she had come to despise) was making a fool of himself, part of her was relieved that he wasn't actually cheating on her (although he obviously acted like he was, she knew it was just an act).
Clegg was a man with Single-Target Sexuality.
He clearly loved his wife Edith very much and was evidently a faithful husband; since they probably both grew up in the same small Yorkshire town, he likely only ever had eyes for her (as borne out by his portrayal in the prequel in which the young Clegg shows little interest in girls). After she died, he simply was not interested in pursuing any sort of relationship with any other woman (even though he was probably not short of offers if Marina's attitude towards him was anything to go by), becoming Heterosexual Life-Partners with Compo instead.Wallace & Gromit's adventures were in fact a series of vivid dreams that Clegg had over the years.
After he'd eaten too much cheese, of course. Wallace being an inventor stemmed from Clegg spending too much time in Seymour's company.