A choice response rife with oxymorons:

Yes it might be the case that those men are straight, actually. Heterosexuality is being attracted to someone of the opposite gender.

Although the men who claim to have attraction towards trans-women can have varied identities and self identifiers, it's mostly men who identify as heterosexual (and usually date or register attraction towards cis women)

The obvious reason ? trans-women are women. No matter what the anatomy of genitals, majority of the men who claim to have almost exclusive or somewhat exclusive same gender attraction are not attracted to the opposite gender. Hence, not to trans-women.

Yes, trans-women have penis, but it's attached to a woman. Enjoying being the recipient in penetration (whether oral or anal) has nothing to do with sexual orientation. But with whom it's being performed, that might be indicative of sexual orientation. There are absolutely, men, who enjoy being pegged by their female partners but are exclusively heterosexual (do not experience attraction to the same gender or a wish to ever engage sexually or romantically with same gender)

It's just the societal emphasis on cis-het penetration as such, that restrictively defines the active (penetrator) and passive (recipient) gender roles and the ubiquitous practise of “gendered-sexuality” Now, having discussed all that, there might be good chances of these men being in denial (discordance). Cuz no matter how far we've progressed, the stigma, hostile prejudice etc. still exists, till date, stopping people from being their authentic selves.