A poster of Greenland's flag is shown on Eric's bedroom wall. This flag was first adopted in 1985 and the episode is set in 1977.
When Eric is bringing in the mail and Jackie tells him that Kelso was also bad his first time, you can clearly see "1 800 CONTACTS" on the brown envelope. This company did not exist until 1995.
The whole cruel dissection of Eric and Donna's "first time" rings totally false. The urge to be deliberately cruel to friends is a result of the Reagan era, not something that was even accepted, let alone encouraged, before then. Cruelty to purported friends was actually shunned, not encouraged, before 1981. Whatever failings might have ensued in sexual matters, for example, were things that might have been discussed in private, but a third party trumpeting such matters in public would generally receive reprobation. Public cruelty is a fashion that came into being with the Reagan era, 1981 and after.