When the bhAgavata speaks of dvija husbands & their wives, it is speaking with reference to the shrauta/smArta dharma that these dvijas followed. The dvija man was expected to do various holy rites, based on shruti & smRti, to the gods & ancestors with his wife. Debasing her through those forbidden sexual acts (her consent is irrelevant; degrading sex even with wife is condemned) would render her unsuitable for sacred rites (which are the essence of dharma). Therefore, with a view to prevent such corrosive acts which perverts the sanctity of dAmpatya, such a hell is threatened for the dvija-s. The restriction to dvija-s is only because the bhAgavatapurANa is speaking with the shrauta-smArta dharma in mind.
shUdra ambiguity
One must always remember the particular historical context in which a text is written. ‘dvija’ refers to brAhmaNa, kSatriya or vaishya, not just brAhmaNa. The terms, ‘advija’ or ‘shUdra’ don’t refer to a monolithic, hereditary group. The 1st obviously cannot refer to a single group, being a negative term (a-dvija, not-dvija is a ‘negative’ term). Even the “positive” term, shUdra did not always refer to the same group in the minds of various authors of texts. It did have different connotations at different points in time.
Going back to what I said previously, “shUdra” has always been a flexible term. manu in one place speaks of there being no pAtaka-s in a shUdra with the commentator saying that apart from acts explicitly forbidden for him (murder, stealing, etc), there are no prohibitions. manu further says that ‘dharma’ (in this context: vratas for devas, bathing in tIrthas, etc) are not compulsory for a shUdra though he isn’t forbidden from that. When one reads that particular shloka in the bhAgavata with this shloka from manu & similar passages, one gets it.
These texts were still functioning from a historically ‘older’ point of view where the dvija-s, bound to perform shrauta-smArta rituals, were expected to conform to very high standards. And for a long long time, the term “shUdra” primarily indicated a loosely defined group. A loosely defined group that consisted of those who didn’t ancestrally practice shrauta-smArta rites & barely had personal restrictions in regards to food, alcohol or sexual habits. That shloka in bhAgavata had simply taken this state of affairs & thus speaks of just dvijas.
shUdra inclusion
But this state of affairs was not permanent. By the time of the composition of the bhAgavata itself, many non-dvija groups had enthusiastically embraced various routes in dharma, outside the shrauta-smArta mainstream: the bhAgavata/vaiSnava, shaiva, etc sampradAya-s. Even prior to this, it was probably the case that certain members of non-dvija origin had enthusiastically taken up the company of brAhmaNa teachers who were willing to instruct them in the dharma of RSi-s, the dharma of shruti & smRti. What do you think happened when these groups, who had not previously embraced the codified, moral dictates of the shruti/smRti, came to do so? They naturally made such practice, such moral adherence a hereditary duty.
In the first few centuries of the common era, the Agamika traditions began developing. By the 1000s, both shaiva & vaiSNava Agama swept across TN & by 1300s-1400s, landed shUdra groups were among the biggest & most devout subscribers. These Agamika traditions which swept across TN (Can’t say the same for all Agamika traditions) happened to be traditions which emphasized the moral code of the shruti-s & smRti-s. This was not the only example. You would have the RCM (and before this, other texts too perhaps; others can fill me in) sweeping across uttarabhArata, emphasizing the same decency & morality the smRti-s emphasize upon. Again, the influence of these traditions reached advija groups too. The Agamika traditions, which obtained a huge number of shUdra initiates (the shaiva tradition in TN is in fact almost wholly under the leadership of sacchUdra maTha-s; sacchUdra=sat+shUdra), expected the same, high moral standards of their shUdra followers as they did for all.
And yet in Northern & Western bhArata, you had another brilliant initiative. The rise of shUdradharma texts; dharmashAstra texts written in medieval-late medieval era with a focus on shUdra ritual observances. smArta karma-s which were once seen as something distant, optional & unnecessary for shUdra-s were remodeled for shUdra-s by certain scholarly brAhmaNa-s. Examples: kRSNasheSa, kamalAkarabhaTTa & the famous gagabhaTTa of MH/GJ & raghunandana of Bengal.
To go back to your initial question, I wouldn’t read that shloka of the bhAgavata as excluding non-dvijas from higher moral standards. With greater enthusiasm towards dharmashAstra & sampradAya from all quarters, the expected moral standards would only be higher for all. As a final addendum: what has been our approach so far? We keep opening up new avenues for dhArmika growth & development, all somehow anchored to the spiritual-moral framework of the overarching & hoary shrauta-smArta tradition. The purANas came up with some vratas for shUdras as well as women (strI-s) in general to do independently (either for shiva, viSNu, sUrya, devI etc).
Then, came the Agamika & bhakti traditions (In TN, there was a tight alliance between shaivAgama & shaiva bhakti). Then came the shUdradharma texts, which codified a body of rites for shUdras & resolved contradictory strands in shAstras in respect of the shUdra’s religious standing (contradictory because, as I mentioned, the term shudra has never held a single, stable meaning/connotation).
But these are not all the avenues. The ayyappa tradition in sabarimala is one instance of a tradition which gets bhaktas of every jAti background into a particular moral state of mind. Well, even young women at home who do not observe the pilgrimage enter that state of mind. They observe worship of the deity; spouses maintain a distance from each other during the vrata and even outside the vrata, genuine ayyappa bhaktas maintain certain standards in their moral character & outlook. Thus, we have 1000s of such avenues/tools, 1000s of such traditions to transform the mind (manas) & intellect (buddhi) of a Hindu, regardless of his or her background; to bring them all into line with the dhArmika values that they all ought to observe in any case.