So was Thomas celibate during the time period the series is set in ?

After he got back from the US trip as valet to Robert, he said it was interesting--with an italic tone of voice.
(In that respect, DA is terribly prudish. No one is allowed to be seen in bed with a hint of sex, unless they're married, or Mary, for whom there are no rules. Anyone else like Ethel or Jimmy is caught and punished at once.)

He would have gone to London for the season with the family or simply on his days off. Plus there would have been gay farmers ;)

I always thought they should have shown a scene where Thomas leaves the London house for the evening, it didn't have to show what went on inside, just Thomas entering one of the underground clubs so the audience knows that he's not a monk up at the abbey (no pun intended!) and that although he's not in a relationship he's still 'meeting people'.

I agree that the show could be prudish in that respect. I know it's a 1920's set show but we all know that behind closed doors a lot of things went on and as an audience we're watching what's going on behind closed doors so why be so vague and prudish?

Imposible, any servant would have 1/2 day off, most of the times the sundays in the morning to go to church. They lived in rural Yorkshire, in fact, when they needed to go to somewhere they needed to ask to Carson or Mrs Hughes and tell them what they are going to do and the time that they could come back. The butler and head house keeper were not only in charge of the servants in terms of work, they were also in charge of their moral behaviour, any misconduct of Thomas or another servant would bring shame in the house of the Crawleys. Remember that Mrs Hughes, a fairly liberal person in that time, was angry with Gwen for keeping a typing machine (bought with her own money) in her bedroom.

The fact that the series in the last season show us the servants going to anywhere (when they were in London, is total unrealistic). The Edwardian house to the servants was very similar to live in a monastery or a regiment. So, it was totally imposible that somebody like Thomas would find "love" in those circunstances, he is lucky that as a gay man has a job and he is not being prosecuted, that it was the sad true for gay men or working origins. The rich ones, simply went to other countries (Bertie´s cousin, the gay marquess, was living Tangiers and that place was the play ground for rich gay men in that period), or they organized encounters in privates residents away of the public eye. Going to a "secret" club in London is very very dangerous, the police could go there anytime and arrest those men.

I think that celibacy was forced on Thomas by the circumstances of his work and sexuality. As condorilloclqt points out, servants had very little free time and on their days off could do little more than visit the local village. A servant wouldn't risk any liaison in the village either, because rumours would carry very easily. So Thomas probably had little option other than sullenly hanging around in the local pub on his days off.

In fact, his sexual repression might explain why he is always so angry and bitter. He probably wants others to share in some of the misery that he is in. It also explains why he would make such unwise advances on Jimmy - he was so desperate that he forgot to be cautious.

Wasn´t any servant who didn´t marry or had a spectacular scandal in the house celibate?

I can´t really say how old Daisy is supposed to be at the beginning, but from what we saw she must have been a virgin even at the shows end.
I don´t think there is a way for Carson to have "gotten some" since he arrived at Downton at a Junior Footman, With Mrs. Hughes this is even a plotpoint.