I can see Seriph's issue. While far be it from me to judge on whether you're FTB or not (Far, far, FAR... Let's leave it at that) the issue of relationships IRL is a valid issue to be discussed. There is a bunch of emotional commitment that goes into a relationship that can be broken for reasons which don't have anything (Or at least very little) to do with emotions. How many think that the Executive's feelings towards the secretary are more emotional than "I'm stressed and you've got just... A great pair of buttocks, let's get nasty"? Alright, now how many of you think that the executive's wife will accept that as an excuse when she walks into his office unnanounced to find him and Jennifer halfway through a bottle of marmalade and pipe cleaners scattered all over the desk?
Naturally there's a considerable difference between having a brain-bleach-reaching affair with your secretary and talking dirty with some guy or gal in sweden via a chatroom, but it can still be seen as a violation of whatever unwritten and ridiculously vague contract you two signed when you started going steady/moved in together/married. This is even true if the 'relationship' you're RPing out is entirely non-sexual or just FTB. In some cases in can be especially true, since your partner might decide that if there's no sex imperative then it's an entirely emotional thing (Whether this is true or not) and feel even more betrayed.
Or they could just shake their head and say 'I'm dating a nerd', maybe laugh at you a bit, then move on. Depends on their attitude and general understanding of how RPing relationships can work.
Ultimately my opinion would be that whatever's been cleared with your partner is just fine, but if you're keeping the 'relationship' a secret then it's a little dodgy. I personally don't engage in completely IC romances at all short of some comedic or plot-driven element (But in the latter case I prefer alts/NPCs) because it's not my gig, and don't engage in OOC or OOC/IC mix romances long distance because of logistics issues.
And on the space lesbians aspect: Simple answer is a combination of the following facts
1) It's not all that unusual for men to play women, especially when the only necessary effect on the RP is a pretty avatar on their bio page. Especially especially when the character introduction page notes that the only difference is how others treat you, and knowing that women are generally treated more nicely online than men.
2) These men will rarely be interested in imagining their female characters lusting over a male. If they choose to be in a relationship, it'll be with their actual preferred gender: Female.*
3) Along with the above, lesbians are hot. If it's only an FTB relationship/they're scarily good at getting in character, they'll be willing to overlook the fact that the person behind the avatar is male.
4) Females, by contract, seem to very rarely choose male characters unless they're trying to pass for men OOC. The contrast might be indicative of which is the superior gender, but who knows.
5) Females tend to be more emotionally based than sexually based when compared to men, so if they choose for their characters to be in a relationship it'll be based more based on personality than gender when compared to men.
6) Along with the above, since most 'girls' in eve are played by men it's easy for straight females to ignore the fact that the character theirs is draped over is female when the person behind that character is male. Assuming they have any issue with that in a purely IC relationship in the first place.
*Obviously doesn't apply to gay men. I'm not interested in rewording/making an entire section to deal with my lack of forethought right now, so use your imagination.