I am supervising a bachelor thesis and find the respective student attractive. Would it be ethical/acceptable to get into a relationship with the student after the thesis is completed?
It is neither ethical nor safe to get into any kind of relationship with a current student. Depending on your institution, it may be prohibited conduct. Brown's rule: "You can lust after girls, or you can lust after boys, but you can never lust after students."
Even former students aren't safe. Some years ago a graduate whose senior project I had supervised showed up in my office and announced that she had come to take me to dinner. We began seeing each other, and then she decided to return to our institution for a master's degree. That resulted in a self-initiated, but very uncomfortable, meeting with dean and VPAA.
The ethical, and safe, thing to do -- now, actually -- would be to look for another faculty member to pass this thesis supervision responsibility to, because your objectivity has been, or could be seen to be, compromised by your feelings.
I was given some very good advice once by a senior professor: when contemplating a particular action, imagine what it would look like if described in a newspaper headline.
"Emcor Involved in Romantic Relationship with Student S/He Was Providing Direct Thesis Supervision to"
Doesn't look so good. Compare:
"Emcor Hands Off Thesis Supervision to Colleague before Embarking on Romantic Relationship with Student"
This doesn't mean you can't read a draft of the thesis, or that you can't be a sounding board for the work. It just means that you shouldn't be the thesis supervisor.
That's how I see it. However, if you are in any doubt, do check with a department administrator.
It is not ethical nor acceptable for you to do. After the supervision is technically over you still have power over the student, such as through reference letters.