At the very beginning of Buddhism, when the Buddha taught hist 1st sermon (SN 56.11) about the Four Noble Truths, which included the Noble Eightfold Path, it was taught the Noble Eightfold Path or Middle Way is for those who have left the household life. To quote:
Bhikkhus, these two extremes should not be followed by one who has gone forth into homelessness. What two? The pursuit of sensual happiness in sensual pleasures, which is low, the way of worldlings, ignoble, unbeneficial; and the pursuit of self-mortification, which is painful, ignoble, unbeneficial. Without veering towards either of these extremes, the Tathagata has awakened to the middle way, which gives rise to vision, which gives rise to knowledge, which leads to peace, to direct knowledge, to enlightenment, to Nibbāna.
And what, bhikkhus, is that middle way awakened to by the Tathagata, which gives rise to vision … which leads to Nibbāna? It is this Noble Eightfold Path; that is, right view, right intention, right speech, right action, right livelihood, right effort, right mindfulness, right concentration. This, bhikkhus, is that middle way awakened to by the Tathagata, which gives rise to vision, which gives rise to knowledge, which leads to peace, to direct knowledge, to enlightenment, to Nibbāna.
Therefore, it seems obvious that Right Mindfulness was never originally intended to be practised with sexual activity because the core essence of Right Mindfulness, as defined (say in SN 45.8), is :
He/she dwells… mindful, having removed covetousness (liking) and displeasure (disliking) in regard to the world. This is called right mindfulness. SN 45.8
At a later time, the sutta MN 117 was introduced, which seems to include a (mundane) eigthfold path for householders (non-monks). MN 117 seems to state right mindfulness can be practised with sexual activity, which means to not practise sexual misconduct when engaging is sex. To quote:
And what, bhikkhus, is right action that is affected by taints (defilements), partaking of merit (morality), ripening in the acquisitions (attachment)? Abstinence from killing living beings, abstinence from taking what is not given, abstinence from misconduct in sensual pleasures: this is right action that is affected by taints…ripening in the acquisitions.
Mindfully one abandons wrong action, mindfully one enters upon and dwells in right action: this is one’s right mindfulness.
In summary, right mindfulness in relation to sexual activity seems to mean to remember to not engage in sexual misconduct.
‘Mindfulness’ is defined as ‘to remember’ or ‘recollection’ (e.g. SN 48.10).
‘Sexual misconduct’ is most basically defined (in AN 10.176) as having sex with people in existing relationships, such as another person’s husband, wife, fiancee, partner; a child/teenager that lives with their parents (particularly if the parents disapprove of the sexual activity); a person that lives in the care of an institution; a person that is under the protection of the law (e.g. a minor under the age of sexual consent or a person protected by rape laws).