It's not so much that the"onlooker" thinks them to be straight, it's that there is a fairly large proportion of men who would describe themselves as straight while having sex with other men - so much so that people in the public health sector working to reduce risky sexual behaviours have a phrase "men who have sex with men" to try to get across to these men that they need to use condoms whether they think of themselves as gay or straight.
However, this feels to me like a derail. Whether or not the men having sex with transwomen who still have a penis think of themselves as gay or straight, the fact remains that for the most part they are under no social pressure to engage in this sort of sex. A lot of young lesbians, in contrast, are being threatened with ostracism and told they are bigots if they won't accept penis.
It's not about what's going into which orifices, but whether that's taking place under coercion, and whether that coercion is applied to women rather than to men.