If I can share my point of view:
Yes without knowledge and approval of partner, he is physically cheating even though no feeling

Cheat could be for few reason.

1. Lust (if you quietly go spa, texting or fling but nothing serious with intention to sleep with the person)

2. Money (If as a guy or girl attracted by money, wealth, gift or sense of financial security which suits her lifestyle or expectation)

3. Attention (Feeling wanted, feeling adore, feeling loved which not so sure will last how long or what is the actual motive)

4. Other advantage (Normally happen in work environment or business which to trade sex or fling to obtain deal, contract or promotion)

5. Pure Bitch (Girl / Guy who feel challenged or attracted to steal or take someone elses partner

6. Power but Coward (Act so good husband, loving father but the other dark side of him date or do something which 180 degree from his daily image)

7. Fairy tale & Imagination (Watching to much movie (K-drama, Porn, etc or imagining their perfect version of partner and keep on finding it without considering the consequences)

8. Pure Stupid (Simply because the person stupid, e.g: fall in love with online scammer until trsf money)

9. Trapped in situation (purposely trap partner treat the person and purposely lead him/her to cheat because taking things for granted, abuse mental & physical, etc)

10. Real Love & Fated (It is fated and it is the better partner, normally this will lead with divorce but the initial started while still in relationship so consider cheat)

Maybe he wish to prevent it to happen to himself or people around him.