The sexually transmitted part is kinda just homophobia. The study they cite specifically sought to compare Men who have Sex with Men and everyone else, and came to the conclusion that the sexually transmitted shigella is far more likely among MSM than any other group and is an endemic sexually transmissible illness among MSM, and blames PrEP and PEP (almost exclusive to the gay and bisexual male community) along with higher treatment rates for other sexually transmitted illnesses like gonorrhea among MSM for the rise in resistant sexually transmitted shigella. They also concluded that sexually transmitted shigellosis among MSM is responsible for introducing more infections to the general public.

https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(26)00227-6/fulltext

Background Shigellosis, caused by Shigella bacteria, is a leading cause of diarrhoeal disease globally. In the last two decades, Shigella circulation in high-income countries has expanded from being a sporadic travel-associated illness to also being an endemic sexually transmissible illness among men who have sex with men (MSM). We aimed to characterise the nature and drivers of this multimodal Shigella transmission in a high-income setting.

And they made a LOT of assumptions to get the results. Basically if they didn't have travel information on a sample, they assumed that it was a non-travel risk. And if they didn't have sex information on a sample, they assumed it was female.

Results S sonnei sequences (n=3514) were classified into three demographic groups based on their potential association with STN (appendix 2 tab 2). Isolates collected on or after March 1, 2020 (85 [2·3%] of 3642) and with missing data on sex (26 [0·7%] of 3557) or age (ten [0·3%] of 3531) were excluded. We assumed that cases with missing or unknown travel data (1795 [51·1%] of 3514) did not have recent high-risk travel10 and that cases with unknown information on sex were female (16 [0·5%] of 3514). We identified 1197 (34·1%) isolates as being from pMSM, 1048 (29·8%) as travel, and 1269 (36·1%) from non-pMSM (949 [75%] female and 320 [25%] male; table; figure 1A; appendix 1 p 8). The urban population centres of London and Manchester had higher proportions of pMSM cases than less populated regions (0·44 [95% CI 0·41–0·46] vs 0·27 [0·25–0·29], Pearson's χ2 test p<0·0001; figure 1B). Antimicrobial resistance against any one of the clinically relevant ciprofloxacin, azithromycin, and ceftriaxone antimicrobials, was highest among pMSM (69·4%, 831 of 1197) compared with antimicrobial resistance among non-pMSM (39·6%, 502 of 1269) and travel cases (48·9%, 512 of 1048; figure 1C; appendix 1 p 9; appendix 2 tab 2).

However, we also found that the intensified transmission among pMSM scaled not only with population size but was also attributable to a greater number of introductions into the population (ie, increased transmission chains). This greater importation of Shigella into sexual transmission networks might reflect increased network density (ie, greater number of sexual contacts among gay, bisexual, and other MSM24) or enhanced biological susceptibility of pMSM to Shigella, possibly associated with HIV (or both).5

In the coming years, it is likely that the spread of sexually transmissible drug-resistant shigellosis will increase. Particularly with the widespread availability of pre-exposure and post-exposure prophylaxis regimens in sexual health practice, including HIV and doxycycline pre-exposure and post-exposure prophylaxis, facilitating sexual behaviours and antimicrobial selection pressure, which can lead to increases in transmission and antimicrobial resistance emergence.18,32,33 It is also likely that our findings of rapid spatial spread and intensified transmission of Shigella among pMSM are occurring for the array of other enteric illnesses that are otherwise known to transmit sexually among pMSM, which might also be increasing.34